Display device

ABSTRACT

A display device includes a pixel portion in which a pixel electrode layer is arranged in a matrix, and an inverted staggered thin film transistor having a combination of at least two kinds of oxide semiconductor layers with different amounts of oxygen is provided corresponding to the pixel electrode layer. In the periphery of the pixel portion in this display device, a pad portion is provided to be electrically connected to a common electrode layer formed on a counter substrate through a conductive layer made of the same material as the pixel electrode layer. One objection of our invention to prevent a defect due to separation of a thin film in various kinds of display devices is realized, by providing a structure suitable for a pad portion provided in a display panel.

TECHNICAL FIELD

The present invention relates to a display device using an oxidesemiconductor and a method for manufacturing the same.

BACKGROUND ART

As typically seen in a liquid crystal display device, a thin filmtransistor formed on a flat plate such as a glass substrate ismanufactured using amorphous silicon or polycrystalline silicon. A thinfilm transistor using amorphous silicon has a low field effect mobility,but can be formed on a large glass substrate. On the other hand, a thinfilm transistor using crystalline silicon has a high field effectmobility, but cannot always be formed on a large glass substrate becauseof a need of a crystallization process such as laser annealing.

In view of the foregoing, techniques in which a thin film transistor ismanufactured using an oxide semiconductor and applied to electronicdevices or optical devices have attracted attention. Examples of thetechniques are disclosed in Patent Document 1 and Patent Document 2,where a thin film transistor is manufactured with zinc oxide or anIn—Ga—Zn—O based oxide semiconductor used for an oxide semiconductorfilm and is used as a switching element or the like of an image displaydevice.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF INVENTION

The field effect mobility of a thin film transistor using an oxidesemiconductor in a channel formation region is higher than that of athin film transistor using amorphous silicon. The oxide semiconductorfilm can be formed by sputtering or the like at a temperature of 300° C.or lower. Its manufacturing process is easier than that of a thin filmtransistor using polycrystalline silicon.

Such an oxide semiconductor is expected to be used for forming a thinfilm transistor on a glass substrate, a plastic substrate, or the like,and to be applied to a liquid crystal display device, anelectroluminescent display device, electronic paper, or the like.

The oxide semiconductor thin film transistor has superior operatingcharacteristics and can be manufactured at a low temperature. However,in order to make efficient use of these features, the structure andmanufacturing conditions of elements need to be optimized, and it isalso necessary to consider the structure of wirings needed to input andoutput signals and the connection structure of the wirings. Although anoxide semiconductor film can be formed at a low temperature, a productcan be defective if a thin film of a metal or the like used for a wiringor an electrode, or an insulating film such as an interlayer insulatingfilm, is separated. In addition, if the connection resistance ofelectrodes in a common connection portion provided on an elementsubstrate side of a display panel is high, there is a problem that spotsappear on a display screen and thus the luminance is decreased.

An object of an embodiment of the present invention is to provide astructure suitable for a common connection portion provided in a displaypanel.

Another object of an embodiment of the present invention is to prevent adefect due to the separation of a thin film in various kinds of displaydevices that are manufactured using stacked layers of an oxidesemiconductor, an insulating film, and a conductive film.

According to an embodiment of the present invention, a display deviceincludes a pixel portion in which a scan line and a signal line crosseach other and a pixel electrode layer is arranged in a matrix, and aninverted staggered thin film transistor having a combination of at leasttwo kinds of oxide semiconductor layers with different amounts of oxygenis provided corresponding to the pixel electrode layer. In the peripheryof the pixel portion in this display device, a pad portion is providedto be electrically connected to a common electrode layer facing thepixel electrode layer through a conductive layer made of the samematerial as the scan line and the signal line.

According to an exemplary mode of the present invention, a displaydevice includes a pixel portion having a thin film transistor connectedto a pixel electrode, and a pad portion electrically connected to acommon electrode facing the pixel electrode, and the display deviceincludes the following structures.

In the pixel portion, a scan line and a signal line cross each other anda pixel electrode layer is arranged in a matrix. The thin filmtransistor is provided corresponding to the pixel electrode layer, andincludes a gate electrode layer connected to the scan line, a gateinsulating layer covering the gate electrode layer, a first oxidesemiconductor layer to be a channel formation region, a second oxidesemiconductor layer over the first oxide semiconductor layer, which isto be a source region and a drain region, and a source electrode layerand a drain electrode layer over the first oxide semiconductor layer andthe second oxide semiconductor layer.

The pad portion is provided in the periphery of the pixel portion, andincludes an insulating layer formed using the same layer as the gateinsulating layer, a conductive layer over the insulating layer, which isformed using the same layer as the source electrode layer and the drainelectrode layer, and an interlayer insulating layer over the conductivelayer. The pad portion can be electrically connected to a commonelectrode layer facing the pixel electrode layer through an opening inthe interlayer insulating layer.

According to an exemplary mode of the present invention, the pad portionprovided in the periphery of the pixel portion may have anotherstructure: a first conductive layer formed using the same layer as thegate electrode layer, an insulating layer formed using the same layer asthe gate insulating layer, and a second conductive layer formed usingthe same layer as the source electrode layer and the drain electrodelayer are stacked in this order. The pad portion can be electricallyconnected to a common electrode layer facing the pixel electrode layerthrough an opening in an interlayer insulating layer provided over thesecond conductive layer.

In the aforementioned structures, the pad portion may have a structurein which an oxide semiconductor layer formed using the same layer as thesecond oxide semiconductor layer is provided between the insulatinglayer formed using the same layer as the gate insulating layer and theconductive layer (or the second conductive layer).

The oxide semiconductor layer (the first oxide semiconductor layer) usedas a channel formation region of the semiconductor layer has a higheroxygen concentration than the oxide semiconductor layer (the secondoxide semiconductor layer) used as a source region and a drain region.It can be said that the first oxide semiconductor layer is anoxygen-excess oxide semiconductor layer and the second oxidesemiconductor layer is an oxygen-deficient oxide semiconductor layer.

The second oxide semiconductor layer has n-type conductivity and has ahigher electric conductivity than the first oxide semiconductor layer.Therefore, the source region and the drain region using the second oxidesemiconductor layer have a lower resistance than the semiconductor layerusing the first oxide semiconductor layer.

The first oxide semiconductor layer has an amorphous structure, and thesecond oxide semiconductor layer includes a crystal grain (nanocrystal)in the amorphous structure in some cases. Note that the crystal grain(nanocrystal) in the second oxide semiconductor layer has a diameter of1 nm to 10 nm, and typically about 2 nm to 4 nm.

Note that ordinal numbers such as “first” and “second” in thisspecification are used for convenience. Therefore, they do not denotethe order of steps, the stacking order of layers, and particular nameswhich specify the invention.

As the first oxide semiconductor layer to be a channel formation regionand/or the second oxide semiconductor layer to be a source region and adrain region, an oxide semiconductor film containing In, Ga, and Zn canbe used. Any one of the elements In, Ga, and Zn may be substituted bytungsten, molybdenum, titanium, nickel, or aluminum.

In this specification, a semiconductor layer formed using an oxidesemiconductor film containing In, Ga, and Zn is also referred to as an“IGZO semiconductor layer”. The IGZO semiconductor layer is anon-single-crystal semiconductor layer and includes as least anamorphous component.

A substrate having a surface on which a pixel electrode layer and a thinfilm transistor electrically connected to the pixel electrode layer areformed is fixed to a counter substrate with an adhesive called asealant.

In a liquid crystal display device, a liquid crystal material is sealedbetween two substrates with a sealant.

The sealant is mixed with a plurality of conductive particles (such asparticles plated with gold), whereby a counter electrode (also referredto as a common electrode) provided on the counter substrate iselectrically connected a common electrode or a common potential line onthe other substrate.

The common potential line can be formed over the same substrate throughthe same manufacturing process as the thin film transistor.

In addition, a portion where the common potential line overlaps theconductive particles in the sealant can be called a common connectionportion. The portion where the common potential line overlaps theconductive particles can also be called a common electrode.

The common potential line formed over the same substrate as the thinfilm transistor can be referred to as a line to supply a voltage to beused as a reference when a liquid crystal is driven by an alternatingcurrent.

In addition to the common potential line connected to the counterelectrode, a capacitor wiring connected to one electrode of a storagecapacitor can be regarded as a variation of the common potential lineand formed over the same substrate as the thin film transistor in asimilar way.

A display device using an electrophoretic display element, which is alsoreferred to as electronic paper, has a structure in which whiteparticles, black particles having a polarity opposite to the whiteparticles, and a dispersion medium (gas or liquid) for dispersing themare included between a pair of substrates. An electrode provided overone of the pair of substrates is a common electrode. Pixel electrodesare provided over the other substrate so as to face the commonelectrode, and a plurality of thin film transistors electricallyconnected to the pixel electrodes are also arranged over the substrate.In the operation of the display device using an electrophoretic displayelement, for example, a voltage positive to the common potential appliedto the common electrode is applied to the pixel electrode for turning awhite display to a black display; a voltage negative to the commonpotential applied to the common electrode is applied to the pixelelectrode for turning the black display to the white display; and thepixel electrode for not changing the display is set at the samepotential as the common potential.

The common potential line formed over the same substrate as the thinfilm transistor can be referred to as a line to supply a voltage to beused as a reference when the electrophoretic display element isoperated.

Note that the display device using an electrophoretic display elementincludes a plurality of separated spaces of a uniform size formed by thepair of substrates and partitions provided between the pair ofsubstrates. A separated space serves as a pixel unit for displaying partof an image. A separated space includes a plurality of white particles,black particles having a polarity opposite to the white particles, and adispersion medium (gas or liquid) for dispersing them.

Also in the display device using an electrophoretic display element, aplurality of colored particles having different polarities and thedispersion medium for dispersing them are sealed between the twosubstrates with a sealant. In addition, in the display device using anelectrophoretic display element, a common electrode provided over onesubstrate and a common potential line formed over the other substrateare electrically connected through conductive particles in a commonconnection portion.

A plastic film can be used as a material for the pair of substrates usedin the liquid crystal display device or the display device using anelectrophoretic display element, depending on a temperature of themanufacturing process.

The gate insulating layer, the first oxide semiconductor layer to be achannel formation region, the second oxide semiconductor layer to be asource region and a drain region, and the source electrode layer and thedrain electrode layer may be formed by sputtering (a sputter method).

Examples of sputtering include an RF sputtering in which ahigh-frequency power source is used for a sputtering power source, a DCsputtering, and a pulsed DC sputtering in which a bias is applied in apulsed manner. The RF sputtering is mainly used in the case of formingan insulating film, and the DC sputtering is mainly used in the case offorming a metal film.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can bedeposited to be stacked in the same chamber, or a plurality of kinds ofmaterials can be deposited by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method by sputtering, there are also areactive sputtering in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a thin compound film thereof, and a bias sputtering in whichvoltage is also applied to a substrate during deposition.

By any of these varieties of sputtering methods, the gate insulatinglayer, the semiconductor layer, the source region and the drain region,and the source electrode layer and the drain electrode layer are formed.

In the case where an IGZO semiconductor layer is used as the first oxidesemiconductor layer (the oxygen-excess oxide semiconductor layer) andthe second oxide semiconductor layer (the oxygen-deficient oxidesemiconductor layer), the first oxide semiconductor layer and the secondoxide semiconductor layer are formed under different depositionconditions. The second oxide semiconductor layer to be a source regionand a drain region is deposited under such conditions that a crystalgrain having a diameter of 1 nm to 10 nm is contained immediately afterthe deposition. For example, if the second oxide semiconductor layer isdeposited by DC sputtering with the use of a target ofIn₂O₃:Ga₂O₃:ZnO=1:1:1 while introducing an argon gas and oxygen at aflow rate ratio of 2:1 or introducing only an argon gas, a filmcontaining a crystal grain having a diameter of 1 nm to 10 nm isobtained in some cases immediately after the deposition. Note that thetarget of In₂O₃:Ga₂O₃:ZnO=1:1:1 is deliberately designed to have such aratio in order to obtain an amorphous oxide semiconductor film.Therefore, the composition ratio of the target may be changed so thatthe crystallinity of the source region and the drain region is furtherimproved. In order to simplify the process and reduce the cost, it ispreferable that the first oxide semiconductor layer to be a channelformation region and the second oxide semiconductor layer to be a sourceregion and a drain region be separately formed by using the same targetand changing only the introduced gas.

A titanium film is preferably formed for the source electrode layer andthe drain electrode layer.

In sputtering, a high energy is applied to a target by Ar ions;therefore, it is considered that a high strain energy exists in thedeposited oxide semiconductor layer (typically, the IGZO semiconductorlayer). In order to release the strain energy, heat treatment ispreferably performed at 200° C. to 600° C., and typically 300° C. to500° C. This heat treatment involves the rearrangement at the atomiclevel. The deposition and heat treatment (including light annealing) areimportant because the strain that inhibits the movement of carriers canbe released by this heat treatment.

Note that the semiconductor devices in this specification indicate allthe devices that can operate by using semiconductor characteristics, andan electro-optical device, a semiconductor circuit, and an electronicappliance are all included in the semiconductor devices.

According to an embodiment of the present invention, a structuresuitable for a pad portion provided in a display panel can be provided.

According to an embodiment of the present invention, the oxidesemiconductor layer and the conductive layer are stacked in the padportion provided in the periphery of the pixel portion, whereby a defectdue to separation of a thin film can be prevented. In addition, byadopting the stacked structure of the oxide semiconductor layer and theconductive layer, the thickness of the pad portion increases and theresistance thereof decreases, resulting in an increase in the strengthof the structure.

According to an embodiment of the present invention, a thin filmtransistor having a small amount of photocurrent, low parasiticcapacitance, a high on-off ratio, and good dynamic characteristics canbe manufactured.

Thus, a display device having high electrical properties and highreliability can be provided according to an embodiment of the presentinvention.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams illustrating a semiconductor device;

FIGS. 2A and 2B are diagrams illustrating a semiconductor device;

FIGS. 3A and 3B are diagrams illustrating a semiconductor device;

FIGS. 4A to 4C are diagrams illustrating a method for manufacturing asemiconductor device;

FIGS. 5A to 5C are diagrams illustrating a method for manufacturing asemiconductor device;

FIG. 6 is a diagram illustrating a method for manufacturing asemiconductor device;

FIG. 7 is a diagram illustrating a method for manufacturing asemiconductor device;

FIG. 8 is a diagram illustrating a method for manufacturing asemiconductor device;

FIG. 9 is a diagram illustrating a semiconductor device;

FIGS. 10A to 10D are diagrams illustrating a semiconductor device;

FIG. 11 is a diagram illustrating a semiconductor device;

FIG. 12 is a diagram illustrating a semiconductor device;

FIGS. 13A and 13B are block diagrams of a semiconductor device;

FIG. 14 is a diagram illustrating a configuration of a signal linedriver circuit;

FIG. 15 is a timing chart illustrating operation of a signal line drivercircuit;

FIG. 16 is a timing chart illustrating operation of a signal line drivercircuit;

FIG. 17 is a diagram illustrating a configuration of a shift register;

FIG. 18 is a diagram illustrating a connection structure of the flipflop illustrated in FIG. 17;

FIG. 19 is an equivalent circuit diagram of a pixel of a semiconductordevice;

FIGS. 20A to 20C are diagrams each illustrating a semiconductor device;

FIGS. 21A to 21C are diagrams illustrating a semiconductor device;

FIG. 22 is a diagram illustrating a semiconductor device;

FIGS. 23A and 23B are diagrams illustrating a semiconductor device;

FIGS. 24A and 24B are views illustrating applications of electronicpaper;

FIG. 25 is an external view illustrating an example of e-book reader;

FIGS. 26A and 26B are external views illustrating a television set and adigital photo frame, respectively;

FIGS. 27A and 27B are external views illustrating examples of anamusement machine;

FIG. 28 is an external view illustrating a cellular phone;

FIGS. 29A and 29B are diagrams illustrating a semiconductor device;

FIGS. 30A and 30B are diagrams illustrating a semiconductor device; and

FIGS. 31A and 31B are diagrams illustrating a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail withreference to drawings. Note that the present invention is not limited tothe description below, and it is apparent to those skilled in the artthat modes and details can be modified in various ways without departingfrom the spirit and scope of the present invention. Accordingly, thepresent invention should not be construed as being limited to thedescription of the embodiments given below. Note that in the structuresof the present invention described below, like portions or portionshaving a similar function are denoted by like reference numerals, andthe description thereof is omitted.

Embodiment 1

This embodiment shows an example of a liquid crystal display device inwhich a liquid crystal layer is sealed between a first substrate and asecond substrate, and a common connection portion (a pad portion) isformed over the first substrate to be electrically connected to acounter electrode provided on the second substrate. Note that a thinfilm transistor is formed as a switching element over the firstsubstrate, and the common connection portion is manufactured in the sameprocess as the switching element in a pixel portion, resulting insimplified process.

The common connection portion is provided in a position overlapping asealant for bonding the first substrate and the second substrate and iselectrically connected to a counter electrode through conductiveparticles in the sealant. Alternatively, the common connection portionis provided in a position which does not overlap the sealant (except forthe pixel portion) and a paste including conductive particles isprovided separately from the sealant so as to overlap the commonconnection portion, whereby the common connection portion can beelectrically connected to the counter electrode through the conductiveparticles in the paste.

FIG. 1A is a cross-sectional view of a semiconductor device in which athin film transistor and a common connection portion are formed over thesame substrate. Note that the thin film transistor illustrated in FIG.1A is an inverted staggered thin film transistor, and includes sourceand drain electrode layers 105 a and 105 b over a semiconductor layer103 with source and drain regions 104 a and 104 b interposedtherebetween. In this embodiment, the semiconductor layer 103 having achannel formation region is a non-single-crystal semiconductor layer (afirst oxide semiconductor layer) containing In, Ga, Zn, and O, andincludes at least an amorphous component. The source and drain regions104 a and 104 b are an oxide semiconductor layer (a second oxidesemiconductor layer) containing In, Ga, Zn, and O, which is formed underdifferent conditions from the IGZO semiconductor layer 103 and has alower oxygen concentration and lower resistance than the semiconductorlayer 103. The source and drain regions 104 a and 104 b has n-typeconductivity and an activation energy (ΔE) of 0.01 eV to 0.1 eV, and canalso be referred to as an n⁺ region. Note that the source and drainregions 104 a and 104 b are a non-single-crystal semiconductor layercontaining In, Ga, Zn, and O, and include at least an amorphouscomponent. Thus, the oxide semiconductor layer used for thesemiconductor layer 103 is an oxygen-excess oxide semiconductor layer,and the oxide semiconductor layer used for the source and drain regionsis an oxygen-deficient semiconductor layer.

When an oxygen-deficient oxide semiconductor layer is provided as thesource and drain regions 104 a and 104 b, a junction between the sourceand drain electrode layers 105 a and 105 b that are metal layers and thesemiconductor layer 103 (the oxygen-excess oxide semiconductor layer) isfavorable and has higher thermal stability than Schottky junction. Inaddition, it is important to positively provide source and drain regionsin order to supply carriers to a channel (on the source side), stablyabsorb carriers from a channel (on the drain side), or preventresistance from occurring at an interface with a source electrode layer(or a drain electrode layer). A reduction in resistance allows goodmobility to be kept even at high drain voltage.

FIG. 1B illustrates an example of a top view of the common connectionportion, and dashed line G1-G2 in FIG. 1B corresponds to a cross sectionof the common connection portion of FIG. 1A. Note that in FIG. 1B,portions similar to those in FIG. 1A are denoted by the same referencenumerals.

The common potential line 185 is provided over the gate insulating layer102 and manufactured of the same material and in the same process as thesource and drain electrode layers 105 a and 105 b.

The common potential line 185 is covered with the protective insulatinglayer 107, and the protective insulating layer 107 has a plurality ofopenings at positions overlapping the common potential line 185. Theseopenings are manufactured in the same process as a contact hole forconnecting the source or drain electrode layer 105 b and the pixelelectrode layer 110.

Note that the contact hole in the pixel portion and the openings in thecommon connection portion are distinctively described because theirsizes differ considerably. In FIG. 1A, the pixel portion and the commonconnection portion are not illustrated on the same scale. For example,the length of dashed line G1-G2 in the common connection portion isabout 500 μm, and the width of the thin film transistor is less than 50μm; thus, the area of the common connection portion is ten times or moreas large as that of the thin film transistor. However, the scales of thepixel portion and the common connection portion are changed in FIG. 1Afor simplification.

The common electrode layer 190 is provided over the protectiveinsulating layer 107 and manufactured of the same material and in thesame process as the pixel electrode layer 110 in the pixel portion.

In this manner, the common connection portion is manufactured in thesame process as the switching element in the pixel portion.

Then, the first substrate 100 provided with the pixel portion and thecommon connection portion is fixed to a second substrate provided with acounter electrode with a sealant.

In the case where the sealant contains conductive particles, the pair ofsubstrates are aligned so that the sealant overlaps the commonconnection portion. For example, in the case of a small liquid crystalpanel, two common connection portions overlap the sealant at oppositecorners of the pixel portion and the like. In the case of a large liquidcrystal panel, four or more common connection portions overlap thesealant.

Note that the common electrode layer 190 is an electrode in contact withthe conductive particles contained in the sealant, and is electricallyconnected to the counter electrode of the second substrate.

In the case of using a liquid crystal injection method, the pair ofsubstrates are fixed with a sealant, and then a liquid crystal isinjected between the pair of substrates. In the case of using a liquidcrystal dropping method, a sealant is drawn on the second substrate orthe first substrate and a liquid crystal is dropped thereon; then, thepair of substrates are bonded to each other under a reduced pressure.

This embodiment shows an example of the common connection portionelectrically connected to the counter electrode. However, the presentinvention is not particularly limited to this example and can be appliedto a connection portion connected to another wiring or a connectionportion connected to an external connection terminal or the like.

For example, in the case of manufacturing a light-emitting displaydevice, unlike a liquid crystal display device, there is no connectionportion to be connected to a counter electrode. Instead, thelight-emitting display device has a portion to connect a cathode(negative electrode) of a light-emitting element to a common wiring, andthe portion may have the same connection structure as that illustratedin FIG. 1A. The cathode of the light-emitting element may have aconnection portion for each pixel. Alternatively, the connection portionmay be provided between a pixel portion and a driver circuit portion.

Embodiment 2

In this embodiment, an example of manufacturing a common connectionportion (a pad portion), in which a wiring formed of the same materialand in the same process as a gate wiring is used as a common potentialline, will be illustrated in FIGS. 2A and 2B.

FIG. 2B illustrates an example of a top view of a common connectionportion, and dashed line E1-E2 in FIG. 2B corresponds to a cross sectionof the common connection portion of FIG. 2A.

Note that as illustrated in FIG. 2A, a thin film transistor in a pixelportion has the same structure as that of Embodiment 1; thus, portionssimilar to those in FIG. 1A are denoted by the same reference numeralsand detailed description is omitted here.

A common potential line 181 is provided over the substrate 100 andmanufactured of the same material and in the same process as a gateelectrode layer 101.

In addition, the common potential line 181 is covered with the gateinsulating layer 102 and the protective insulating layer 107. The gateinsulating layer 102 and the protective insulating layer 107 have aplurality of openings at positions overlapping the common potential line181. These openings, unlike in Embodiment 1, have a large depth whichcorresponds to the thickness of the two insulating layers. Note thatthese openings are manufactured by etching in the same process as acontact hole for connecting the source electrode layer 105 a or drainelectrode layer 105 b and the pixel electrode layer 110, and thenfurther etching the gate insulating layer 102 selectively.

The common electrode layer 190 is provided over the protectiveinsulating layer 107 and manufactured of the same material and in thesame process as the pixel electrode layer 110 in the pixel portion.

In this manner, the common connection portion is manufactured in thesame process as the switching element in the pixel portion.

Then, the first substrate 100 provided with the pixel portion and thecommon connection portion is fixed to a second substrate provided with acounter electrode with a sealant.

In the case where the sealant contains conductive particles, the pair ofsubstrates are aligned so that the sealant overlaps the commonconnection portion.

Note that the common electrode layer 190 is an electrode in contact withthe conductive particles contained in the sealant, and is electricallyconnected to the counter electrode of the second substrate.

In the case of using a liquid crystal injection method, the pair ofsubstrates are fixed with a sealant, and then a liquid crystal isinjected between the pair of substrates. In the case of using a liquidcrystal dropping method, a sealant is drawn on the second substrate orthe first substrate and a liquid crystal is dropped thereon; then, thepair of substrates are bonded to each other under a reduced pressure.

This embodiment shows an example of the common connection portionelectrically connected to the counter electrode. However, the presentinvention is not particularly limited to this example and can be appliedto a connection portion connected to another wiring or a connectionportion connected to an external connection terminal or the like.

Embodiment 3

In this embodiment, an example of manufacturing a common connectionportion (a pad portion), in which an electrode formed of the samematerial and in the same process as a gate wiring is formed and a wiringformed of the same material and in the same process as a sourceelectrode layer is provided as a common potential line over theelectrode, will be illustrated in FIGS. 3A and 3B.

FIG. 3B illustrates an example of a top view of a common connectionportion, and dashed line F1-F2 in FIG. 3B corresponds to a cross sectionof the common connection portion of FIG. 3A.

Note that as illustrated in FIG. 3A, a thin film transistor in a pixelportion has the same structure as that of Embodiment 1; thus, portionssimilar to those in FIG. 1A are denoted by the same reference numeralsand detailed description is omitted here.

A connection electrode layer 191 is provided over the substrate 100 andmanufactured of the same material and in the same process as the gateelectrode layer 101.

In addition, the connection electrode layer 191 is covered with the gateinsulating layer 102 and the protective insulating layer 107. The gateinsulating layer 102 and the protective insulating layer 107 have anopening at a position overlapping the common electrode layer 190. Thisopening, unlike in Embodiment 1, has a large depth which corresponds tothe thickness of the two insulating layers. Note that this opening ismanufactured by etching in the same process as a contact hole forconnecting the source electrode layer 105 a or drain electrode layer 105b and the pixel electrode layer 110, and then further etching the gateinsulating layer 102 selectively.

The common potential line 185 is provided over the gate insulating layer102 and manufactured of the same material and in the same process as thesource and drain electrode layers 105 a and 105 b.

The common potential line 185 is covered with the protective insulatinglayer 107, and the protective insulating layer 107 has a plurality ofopenings at positions overlapping the common potential line 185. Theseopenings are manufactured in the same process as a contact hole forconnecting the source electrode layer 105 a or drain electrode layer 105b and the pixel electrode layer 110.

The common electrode layer 190 is provided over the protectiveinsulating layer 107 and manufactured of the same material and in thesame process as the pixel electrode layer 110 in the pixel portion.

In this manner, the common connection portion is manufactured in thesame process as the switching element in the pixel portion.

Then, the first substrate 100 provided with the pixel portion and thecommon connection portion is fixed to a second substrate provided with acounter electrode with a sealant.

Note that in this embodiment, a plurality of conductive particles areselectively disposed in the opening of the gate insulating layer. Thatis, the plurality of conductive particles are disposed in a region wherethe common electrode layer 190 and the connection electrode layer 191are in contact with each other. The common electrode layer 190 touchingboth the connection electrode layer 191 and the common potential line185 is an electrode in contact with the conductive particles, and iselectrically connected to the counter electrode of the second substrate.

In the case of using a liquid crystal injection method, the pair ofsubstrates are fixed with a sealant, and then a liquid crystal isinjected between the pair of substrates. In the case of using a liquidcrystal dropping method, a sealant is drawn on the second substrate orthe first substrate and a liquid crystal is dropped thereon; then, thepair of substrates are bonded to each other under a reduced pressure.

This embodiment shows an example of the common connection portionelectrically connected to the counter electrode. However, the presentinvention is not particularly limited to this example and can be appliedto a connection portion connected to another wiring or a connectionportion connected to an external connection terminal or the like.

Embodiment 4

In this embodiment, another example of the display device shown inEmbodiment 1, in which source and drain electrode layers and source anddrain regions are formed by etching using the same mask, will beillustrated in FIGS. 29A and 29B.

FIG. 29A is a cross-sectional view of a semiconductor device in which athin film transistor and a common connection portion (a pad portion) aremanufactured over the same substrate. A thin film transistor 172illustrated in FIG. 29A is an inverted staggered thin film transistorand is an example of a thin film transistor in which the source anddrain electrode layers 105 a and 105 b are provided over thesemiconductor layer 103 with the source and drain regions 104 a and 104b interposed therebetween. In the thin film transistor 172, an oxidesemiconductor layer forming the source and drain regions 104 a and 104 band a conductive layer forming the source and drain electrode layers 105a and 105 b are etched using the same mask.

Therefore, in the thin film transistor 172, the source and drainelectrode layers 105 a and 105 b and the source and drain regions 104 aand 104 b have the same shape, and the source and drain regions 104 aand 104 b are placed under the source and drain electrode layers 105 aand 105 b.

Accordingly, also in the common connection portion, an oxidesemiconductor layer 186 manufactured of the same material and in thesame process as the source and drain regions 104 a and 104 b is formedbetween the gate insulating layer 102 and the common potential line 185.

Note that FIG. 29B illustrates an example of a top view of the commonconnection portion, and dashed line G1-G2 in FIG. 29B corresponds to across section of the common connection portion of FIG. 29A.

Note that as illustrated in FIG. 29B, the top view of the commonconnection portion has the same structure as that of Embodiment 1; thus,portions similar to those in FIG. 1B are denoted by the same referencenumerals and detailed description is omitted here.

According to this embodiment, the oxide semiconductor layer and theconductive layer are stacked in the common connection portion (the padportion) provided in the periphery of the pixel portion, whereby adefect due to separation of a thin film can be prevented, resulting inan increase in the strength of the structure. In addition, by adoptingthe stacked structure of the oxide semiconductor layer and theconductive layer, the thickness of the pad portion increases and theresistance thereof decreases.

Embodiment 5

In this embodiment, another example of the display device shown inEmbodiment 3, in which source and drain electrode layers and source anddrain regions are formed by etching using the same mask, will beillustrated in FIGS. 30A and 30B.

FIG. 30A is a cross-sectional view of a semiconductor device in which athin film transistor and a common connection portion (a pad portion) aremanufactured over the same substrate.

Note that as illustrated in FIG. 30A, a thin film transistor in a pixelportion has the same structure as that of Embodiment 4; thus, portionssimilar to those in FIG. 29A are denoted by the same reference numeralsand detailed description is omitted here.

In the thin film transistor 172, an oxide semiconductor layer formingthe source and drain regions 104 a and 104 b and a conductive layerforming the source and drain electrode layers 105 a and 105 b are etchedusing the same mask. Therefore, in the thin film transistor 172, thesource and drain electrode layers 105 a and 105 b and the source anddrain regions 104 a and 104 b have the same shape, and the source anddrain regions 104 a and 104 b are placed under the source and drainelectrode layers 105 a and 105 b.

Also in the common connection portion, the oxide semiconductor layer 186manufactured of the same material and in the same process as the sourceand drain regions 104 a and 104 b is formed between the gate insulatinglayer 102 and the common potential line 185.

FIG. 30B illustrates an example of a top view of a common connectionportion, and dashed line F1-F2 in FIG. 30B corresponds to a crosssection of the common connection portion of FIG. 30A.

Note that as illustrated in FIG. 30B, the top view of the commonconnection portion has the same structure as that of Embodiment 3; thus,portions similar to those in FIG. 3B are denoted by the same referencenumerals and detailed description is omitted here.

According to this embodiment, the oxide semiconductor layer and theconductive layer are stacked in the common connection portion (the padportion) provided in the periphery of the pixel portion, whereby adefect due to separation of a thin film can be prevented, resulting inan increase in the strength of the structure. In addition, by adoptingthe stacked structure of the oxide semiconductor layer and theconductive layer, the thickness of the pad portion increases and theresistance thereof decreases.

Embodiment 6

In this embodiment, a manufacturing process of a display deviceincluding a thin film transistor of one embodiment of the presentinvention will be described with reference to FIGS. 4A to 4C, FIGS. 5Ato 5C, FIGS. 6 to 9, FIGS. 10A to 10D, and FIG. 11.

In FIG. 4A, as the light transmitting substrate 100, it is possible touse a glass substrate made of barium borosilicate glass,aluminoborosilicate glass, or the like typified by #7059 glass, #1737glass, or the like manufactured by Corning Incorporated.

After a conductive layer is formed on the entire surface of thesubstrate 100, a resist mask is formed by a first photolithography step.Then, unnecessary portions are removed by etching, thereby formingwirings and electrodes (a gate wiring including the gate electrode layer101, a capacitor wiring 108, and a first terminal 121). At that time,etching is performed so that at least the edge of the gate electrodelayer 101 is tapered. A cross-sectional view at this stage isillustrated in FIG. 4A. Note that FIG. 6 is a top view at this stage.

The gate wiring including the gate electrode layer 101, the capacitorwiring 108, and the first terminal 121 in the terminal portion arepreferably formed of a low-resistant conductive material such asaluminum (Al) or copper (Cu). However, aluminum alone has thedisadvantages of low heat resistance, being easily corroded, and thelike; thus, it is used in combination with a conductive material havingheat resistance. As the conductive material having heat resistance, itis possible to use an element selected from titanium (Ti), tantalum(Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), andscandium (Sc), an alloy containing any of these elements as itscomponent, an alloy containing a combination of any of these elements,or a nitride containing any of these elements as its component.

Then, the gate insulating layer 102 is formed on the entire surface ofthe gate electrode layer 101. The gate insulating layer 102 is formed toa thickness of 50 nm to 250 nm by sputtering or the like.

For example, as the gate insulating layer 102, a silicon oxide film isformed to a thickness of 100 nm by sputtering. It is needless to saythat the gate insulating layer 102 is not limited to such a siliconoxide film, and other insulating films such as a silicon oxynitridefilm, a silicon nitride film, an aluminum oxide film, or a tantalumoxide film may be used to form a single-layer structure or a multi-layerstructure.

The surface of the gate insulating layer may be cleaned by plasmatreatment before forming an oxide semiconductor layer (an IGZOsemiconductor layer) that is to be a channel formation region. It iseffective to perform plasma treatment to remove dust such as organicsubstances on the surface of the gate insulating layer. It is alsoeffective that the surface of the gate insulating layer is subjected toplasma treatment to be an oxygen-excess region, which serves as anoxygen supply source to modify the interface between the gate insulatinglayer and the IGZO semiconductor layer in heat treatment (200° C. to600° C.) to increase reliability in subsequent steps.

After the plasma treatment, an oxide semiconductor layer is preferablydeposited by sputtering without being exposed to the atmosphere. If adeposition target substrate is exposed to the atmosphere before an oxidesemiconductor layer is deposited, moisture or the like is attached tothe deposition target substrate, which may adversely affect theinterface state leading to variations in threshold values, degradationof electrical properties, production of a normally-on TFT, and the like.The plasma treatment is performed using oxygen gas or argon gas. Insteadof argon gas, other rare gases may be used.

In order to form the IGZO semiconductor layer to be a channel formationregion by a sputtering method without exposure to the air after plasmatreatment, it is preferable to perform a kind of plasma treatment calledreverse sputtering which can be performed in the same chamber as thatused in the formation of the IGZO semiconductor layer, The reversesputtering treatment is a method in which voltage is applied to asubstrate side in an oxygen atmosphere or an oxygen and argon atmospherewithout applying voltage to a target side, so that plasma is generatedto modify the surface of a thin film on the substrate.

In the case where the plasma treatment is performed in an oxygenatmosphere, the surface of the gate insulating layer is exposed tooxygen radicals to be modified into an oxygen-excess region, therebyincreasing the oxygen concentration at the interface with an oxidesemiconductor layer which is deposited later to be a channel formationregion. In other words, if oxygen radical treatment is performed on thegate insulating layer and an oxide semiconductor layer is stackedthereon, and then heat treatment is performed, the oxygen concentrationof the oxide semiconductor layer to be a channel formation region on thegate insulating layer side can also be increased. Accordingly, theoxygen concentration reaches a peak at the interface between the gateinsulating layer and the oxide semiconductor layer to be a channelformation region, and the oxygen concentration of the gate insulatinglayer has a concentration gradient which increases toward the interfacebetween the gate insulating layer and the oxide semiconductor layer tobe a channel formation region. The gate insulating layer including anoxygen-excess region is compatible with the oxide semiconductor layer tobe a channel formation region that is an oxygen-excess oxidesemiconductor layer, so that favorable interface properties can beobtained.

Oxygen radicals may be produced in a plasma generation apparatus withthe use of a gas containing oxygen, or in an ozone generation apparatus.By exposing a thin film to the produced oxygen radicals or oxygen, thesurface of the film can be modified.

The plasma treatment is not limited to one using oxygen radicals, andmay be performed using argon and oxygen radicals. The treatment usingargon and oxygen radicals is treatment in which argon gas and oxygen gasare introduced to generate plasma, thereby modifying the surface of athin film.

Argon atoms (Ar) in a reaction space where an electric field is appliedto generate discharge plasma are excited or ionized by electrons (e) inthe discharge plasma, thereby being converted into argon radicals (Ar*),argon ions (Ar⁺), or electrons (e). Argon radicals (Ar*), which are in ahigh-energy metastable state, react with the peripheral atoms of thesame kind or of different kinds to be returned to a stable state byexciting or ionizing the atoms, whereby a reaction occurs like anavalanche. If oxygen exists in the periphery at that time, oxygen atoms(O) are excited or ionized to be converted into oxygen radicals (O*),oxygen ions (O⁺), or oxygen (O). The oxygen radicals (O*) react with amaterial on the surface of a thin film that is to be processed, so thatthe surface is modified, and the oxygen radicals also react with anorganic substance on the surface, so that the organic substance isremoved. The plasma treatment is thus performed. Note that radicals ofargon gas have the properties of being kept in a metastable state for alonger time than radicals of reactive gas (oxygen gas). Therefore, it isgeneral to use argon gas to generate plasma.

Then, a first oxide semiconductor film (in this embodiment, a first IGZOfilm) is deposited on the gate insulating layer 102. The first IGZO filmis deposited without being exposed to the air after the plasmatreatment, which is advantageous in that dust or moisture is notattached to the interface between the gate insulating layer and thesemiconductor film. Here, with the use of an oxide semiconductor targetcontaining In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1), which has a diameterof 8 inches, deposition is performed in an argon atmosphere or an oxygenatmosphere at a distance between the substrate and the target of 170 mm,a pressure of 0.4 Pa, and a direct current (DC) power supply of 0.5 kW.Note that a pulsed direct current (DC) power supply is preferably usedto reduce dust and obtain a uniform distribution of film thickness. Thethickness of the first IGZO film is 5 nm to 200 nm, and in thisembodiment, the thickness of the first IGZO film is 100 nm.

The gate insulating layer and the first IGZO film can be successivelydeposited by sputtering without being exposed to the air by changing agas introduced into a chamber or a target placed in the chamber asappropriate. When the films are successively deposited without beingexposed to the air, impurities can be prevented from entering the films.In the case where the films are successively deposited without beingexposed to the air, a multi-chamber manufacturing apparatus ispreferably used.

Then, a second oxide semiconductor film (in this embodiment, a secondIGZO film) is deposited by sputtering without being exposed to the air.Here, with the use of a target of In₂O₃:Ga₂O₃:ZnO=1:1:1, sputteringdeposition is performed at a pressure of 0.4 Pa, a power of 500 W, atemperature of room temperature, and an argon gas flow rate of 40 sccm.Although a target of In₂O₃:Ga₂O₃:ZnO=1:1:1 is deliberately used, an IGZOfilm including crystal grains having a diameter of 1 nm to 10 nm isformed in some cases immediately after deposition. It is said that thepresence, density, and diameter of crystal grains can be controlled byadjusting the deposition conditions of reactive sputtering asappropriate, such as the composition ratio of a target, the depositionpressure (0.1 Pa to 2.0 Pa), the power (250 W to 3000 W: 8 inches φ), orthe temperature (room temperature to 100° C.). The diameter of crystalgrains is controlled within a range of 1 nm to 10 nm The thickness ofthe second IGZO film is 5 nm to 20 nm. It is needless to say that, ifthe film includes crystal grains, the diameter of the crystal grainsdoes not exceed the thickness of the film. In this embodiment, thethickness of the second IGZO film is 5 nm.

The first IGZO film and the second IGZO film are deposited underdifferent conditions, so that the oxygen concentration of the first IGZOfilm is higher than that of the second IGZO film. For example, the flowrate ratio of oxygen gas to argon gas under the deposition conditions ofthe first IGZO film is higher than that under the deposition conditionsof the second IGZO film. Specifically, the second IGZO film is depositedin a rare gas (such as argon or helium) atmosphere (or an atmospherecontaining oxygen at 10% or less and an argon gas at 90% or more), andthe first IGZO film is deposited in an oxygen atmosphere (or a flow rateof oxygen gas is equal to or more than a flow rate of argon gas). Whenthe first IGZO film contains more oxygen, the conductivity of the firstIGZO film can be made lower than that of the second IGZO film. Inaddition, the off-current of the first IGZO film can be reduced when thefirst IGZO film contains more oxygen, whereby a thin film transistorhaving a high on/off ratio can be obtained.

The second IGZO film may be deposited in the same chamber as that usedin the preceding reverse sputtering treatment, or may be deposited in adifferent chamber as long as it can be deposited without being exposedto the air.

Then, heat treatment is preferably performed at 200° C. to 600° C., andtypically, 300° C. to 500° C. Here, heat treatment at 350° C. for onehour is performed in a furnace in a nitrogen atmosphere. This heattreatment involves the rearrangement at the atomic level in the IGZOfilm. The heat treatment (including light annealing) in this step isimportant because the strain that inhibits the movement of carriers canbe released. Note that there is no particular limitation on the timingof the heat treatment, and the heat treatment may be performed at anytime after the deposition of the second IGZO film, for example, afterthe formation of a pixel electrode.

Next, a second photolithography step is performed to form a resist mask,and the first IGZO film and the second IGZO film are etched. Here,unnecessary portions are removed by wet etching using ITO07N(manufactured by KANTO CHEMICAL CO., INC.), thereby forming an IGZO film109 that is an oxygen-excess first IGZO film and an IGZO film 111 thatis an oxygen-deficient second IGZO film. Note that this etching step isnot limited to wet etching and dry etching may also be performed. Across-sectional view at this stage is illustrated in FIG. 4B. Note thatFIG. 7 is a top view at this stage.

Then, a conductive film 132 made of a metal material is formed over theIGZO film 109 and the IGZO film 111 by sputtering or vacuum evaporation.A cross-sectional view at this stage is illustrated in FIG. 4C.

As the material of the conductive film 132, there are an elementselected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any ofthese elements as its component, an alloy containing a combination ofany of these elements, and the like. If heat treatment at 200° C. to600° C. is performed, the conductive film preferably has heat resistanceenough to withstand the heat treatment. Since aluminum alone has thedisadvantages of low heat resistance, being easily corroded, and thelike, it is used in combination with a conductive material having heatresistance. As the conductive material having heat resistance which iscombined with aluminum, it is possible to use an element selected fromtitanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium(Cr), neodymium (Nd), and scandium (Sc), an alloy containing any ofthese elements as its component, an alloy containing a combination ofany of these elements, or a nitride containing any of these elements asits component.

Here, the conductive film 132 has a single-layer structure of a titaniumfilm. The conductive film 132 may also have a two-layer structure inwhich a titanium film is stacked on an aluminum film. Alternatively, theconductive film 132 may have a three-layer structure in which a titaniumfilm, an aluminum film containing neodymium (an Al—Nd film), and atitanium film are stacked in order. Further alternatively, theconductive film 132 may have a single-layer structure of an aluminumfilm containing silicon.

Next, a third photolithography step is performed to form a resist mask131, and unnecessary portions are removed by etching, thereby formingthe source and drain electrode layers 105 a and 105 b and the source anddrain regions 104 a and 104 b. This etching step is performed by wetetching or dry etching. For example, in the case where an aluminum filmor an aluminum alloy film is used as the conductive film 132, wetetching can be performed using a solution in which phosphoric acid,acetic acid, and nitric acid are mixed. Here, with the use of an ammoniahydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), theconductive film 132 made of titanium is wet-etched to form the sourceand drain electrode layers 105 a and 105 b, and the IGZO film 111 iswet-etched to form the source and drain regions 104 a and 104 b. In thisetching step, an exposed region of the IGZO film 109 is partly etched tobe the semiconductor layer 103. Thus, a channel region of thesemiconductor layer 103 between the source and drain regions 104 a and104 b has a small thickness. In FIG. 5A, the source and drain electrodelayers 105 a and 105 b and the source and drain regions 104 a and 104 bare simultaneously etched using an ammonia hydrogen peroxide mixture;therefore, the edges of the source and drain electrode layers 105 a and105 b are aligned with the edges of the source and drain regions 104 aand 104 b to have a continuous structure. In addition, wet etchingallows the layers to be etched isotropically, so that the edges of thesource and drain electrode layers 105 a and 105 b are recessed from theresist mask 131. Through the above steps, a thin film transistor 170including the IGZO semiconductor layer 103 as a channel formation regioncan be manufactured. A cross-sectional view at this stage is illustratedin FIG. 5A. Note that FIG. 8 is a top view at this stage.

Furthermore, the exposed channel formation region of the semiconductorlayer 103 may be subjected to oxygen radical treatment, so that anormally-off thin film transistor can be obtained. In addition, theradical treatment can repair damage due to the etching of thesemiconductor layer 103. The radical treatment is preferably performedin an atmosphere of O₂ or N₂O, and preferably an atmosphere of N₂, He,or Ar each containing oxygen. The radical treatment may also beperformed in an atmosphere in which Cl₂ or CF₄ is added to the aboveatmosphere. Note that the radical treatment is preferably performed withno bias applied.

In the third photolithography step, a second terminal 122 that is madeof the same material as the source and drain electrode layers 105 a and105 b remains in the terminal portion. Note that the second terminal 122is electrically connected to a source wiring (a source wiring includingthe source and drain electrode layers 105 a and 105 b).

If using a resist mask having a plurality of regions with differentthicknesses (typically, two kinds of thicknesses) formed using amulti-tone mask, the number of resist masks can be reduced, resulting insimplified process and lower cost.

Next, the resist mask 131 is removed and the protective insulating layer107 is formed to cover the thin film transistor 170. The protectiveinsulating layer 107 can be formed of a silicon nitride film, a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, atantalum oxide film, or the like which is obtained by sputtering.

Then, a fourth photolithography step is performed to form a resist mask,and the protective insulating layer 107 is etched to form a contact hole125 reaching the source electrode layer 105 a or drain electrode layer105 b. In addition, a contact hole 127 reaching the second terminal 122is also formed in the same etching step. In order to reduce the numberof masks, the gate insulating layer is preferably etched using the sameresist mask so that a contact hole 126 reaching the gate electrode isformed using the same resist mask. A cross-sectional view at this stageis illustrated in FIG. 5B.

Next, the resist mask is removed, and then a transparent conductive filmis formed. The transparent conductive film is formed of indium oxide(In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂, abbreviated to ITO),or the like by sputtering, vacuum evaporation, or the like. Such amaterial is etched with a hydrochloric acid-based solution. However,since a residue is easily generated particularly in etching ITO, indiumoxide-zinc oxide alloy (In₂O₃—ZnO) may be used to improve etchingprocessability.

Next, a fifth photolithography step is performed to form a resist mask,and unnecessary portions are removed by etching, thereby forming thepixel electrode layer 110.

In the fifth photolithography step, a storage capacitor is formedbetween the capacitor wiring 108 and the pixel electrode layer 110 byusing the gate insulating layer 102 and the protective insulating layer107 in the capacitor portion as a dielectric.

Furthermore, in the fifth photolithography step, the first terminal andthe second terminal are covered with the resist mask so that transparentconductive films 128 and 129 remain in the terminal portion. Thetransparent conductive films 128 and 129 serve as an electrode or awiring connected to an FPC. The transparent conductive film 129 formedover the second terminal 122 is a connecting terminal electrode servingas an input terminal of a source wiring.

Then, the resist mask is removed. A cross-sectional view at this stageis illustrated in FIG. 5C. Note that FIG. 9 is a top view at this stage.

FIGS. 10A to 10D are respectively a cross-sectional view and a top viewof a gate wiring terminal portion at this stage. FIG. 10A is across-sectional view taken along line C1-C2 of FIG. 10B. In FIG. 10A, atransparent conductive film 155 formed over the protective insulatingfilm 154 is a connecting terminal electrode serving as an inputterminal. Furthermore, in the terminal portion of FIG. 10A, a firstterminal 151 made of the same material as the gate wiring and aconnection electrode layer 153 made of the same material as the sourcewiring overlap each other with a gate insulating layer 152 interposedtherebetween, and are electrically connected to each other through thetransparent conductive film 155. Note that a part of FIG. 5C where thetransparent conductive film 128 is in contact with the first terminal121 corresponds to a part of FIG. 10A where the transparent conductivefilm 155 is in contact with the first terminal 151.

FIGS. 10C and 10D are respectively a cross-sectional view and a top viewof a source wiring terminal portion which is different from thatillustrated in FIG. 5C. FIG. 10C is a cross-sectional view taken alongline D1-D2 of FIG. 10D. In FIG. 10C, the transparent conductive film 155formed over the protective insulating film 154 is a connecting terminalelectrode serving as an input terminal. Furthermore, in the terminalportion of FIG. 10C, an electrode layer 156 made of the same material asthe gate wiring is formed under a second terminal 150 electricallyconnected to the source wiring and overlaps the second terminal 150 withthe gate insulating layer 152 interposed therebetween. The electrodelayer 156 is not electrically connected to the second terminal 150, anda capacitor to prevent noise or static electricity can be formed if thepotential of the electrode layer 156 is set to a potential differentfrom that of the second terminal 150, such as floating, GND, or 0 V. Thesecond terminal 150 is electrically connected to the transparentconductive film 155 with an opening of the protective insulating film154 interposed therebetween.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided depending on the pixel density. Also in the terminal portion,the first terminal at the same potential as the gate wiring, the secondterminal at the same potential as the source wiring, the third terminalat the same potential as the capacitor wiring, and the like are eacharranged in plurality. There is no particular limitation on the numberof each of the terminals, and the number of the terminals may bedetermined by a practitioner as appropriate.

Through these five photolithography steps, a pixel thin film transistorportion including the thin film transistor 170 which is a bottom-gaten-channel thin film transistor, and the storage capacitor can becompleted using the five photomasks. When these pixel thin filmtransistor portion and storage capacitor are arranged in a matrixcorresponding to respective pixels, a pixel portion can be formed andone of the substrates for manufacturing an active matrix display devicecan be obtained. In this specification, such a substrate is referred toas an active matrix substrate for convenience.

In the case of manufacturing an active matrix liquid crystal displaydevice, an active matrix substrate and a counter substrate provided witha counter electrode are bonded to each other with a liquid crystal layerinterposed therebetween. Note that a common electrode electricallyconnected to the counter electrode on the counter substrate is providedover the active matrix substrate, and a fourth terminal electricallyconnected to the common electrode is provided in the terminal portion.The fourth terminal is provided so that the common electrode is fixed toa predetermined potential such as GND or 0 V.

One embodiment of the present invention is not limited to the pixelstructure of FIG. 9, and an example of the top view different from FIG.9 is illustrated in FIG. 11. FIG. 11 illustrates an example in which acapacitor wiring is not provided and a pixel electrode layer overlaps agate wiring of an adjacent pixel with a protective insulating film and agate insulating layer interposed therebetween to form a storagecapacitor. In that case, the capacitor wiring and the third terminalconnected to the capacitor wiring can be omitted. Note that in FIG. 11,portions similar to those in FIG. 9 are denoted by the same referencenumerals.

In an active matrix liquid crystal display device, pixel electrodesarranged in a matrix are driven to form a display pattern on a screen.Specifically, voltage is applied between a selected pixel electrode anda counter electrode corresponding to the pixel electrode, so that aliquid crystal layer provided between the pixel electrode and thecounter electrode is optically modulated and this optical modulation isrecognized as a display pattern by an observer.

In displaying moving images, a liquid crystal display device has aproblem that a long response time of liquid crystal molecules themselvescauses afterimages or blurring of moving images. In order to improve themoving-image characteristics of a liquid crystal display device, adriving method called black insertion is employed in which black isdisplayed on the whole screen every other frame period.

Further, there is another driving technique which is so-calleddouble-frame rate driving. In the double-frame rate driving, a verticalcycle is set 1.5 times as much as a normal vertical cycle or more(preferably 2 times or more), whereby moving image characteristics areimproved.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving method maybe employed in which a plurality of LEDs (light-emitting diodes) or aplurality of EL light sources are used to form a surface light source asa backlight, and each light source of the surface light source isindependently driven in a pulsed manner in one frame period. As thesurface light source, three or more kinds of LEDs may be used and an LEDemitting white light may be used. Since a plurality of LEDs can becontrolled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. According to this driving method, LEDs can bepartly turned off; therefore, an effect of reducing power consumptioncan be obtained particularly in the case of displaying an image having alarge part on which black is displayed.

By combining these driving methods, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared to those of conventional liquid crystal displaydevices.

The n-channel transistor of this embodiment includes an IGZOsemiconductor layer in a channel formation region and has good dynamiccharacteristics. Accordingly, these driving methods can be applied incombination to the n-channel transistor of this embodiment.

In the case of manufacturing a light-emitting display device, oneelectrode (also referred to as a cathode) of an organic light-emittingelement is set to a low power supply potential such as GND or 0 V; thus,a terminal portion is provided with a fourth terminal for setting thecathode to a low power supply potential such as GND or 0 V. Also in thecase of manufacturing a light-emitting display device, a power supplyline is provided in addition to a source wiring and a gate wiring.Accordingly, a terminal portion is provided with a fifth terminalelectrically connected to the power supply line.

If a gate electrode layer, a gate insulating layer, a semiconductorlayer (an oxygen-excess oxide semiconductor layer containing In, Ga, andZn), and source and drain electrode layers are stacked without providingsource and drain regions (oxygen-deficient oxide semiconductor layerscontaining In, Ga, and Zn), the distance between the gate electrodelayer and the source and drain electrode layers is reduced to increasethe parasitic capacitance therebetween. In addition, the parasiticcapacitance is further increased by a reduction in the thickness of thesemiconductor layer. In this embodiment, a thin film transistor has astacked structure in which a gate electrode layer, a gate insulatinglayer, a semiconductor layer, source and drain electrode regions, andsource and drain electrode layers are stacked, parasitic capacitance canbe suppressed even when the thickness of the semiconductor layer issmall.

According to this embodiment, a thin film transistor having a smallamount of photocurrent, low parasitic capacitance, a high on-off ratio,and good dynamic characteristics can be manufactured. Thus, asemiconductor device including a thin film transistor having highelectrical properties and high reliability can be provided.

Embodiment 7

In this embodiment, another example of the display device shown inEmbodiment 3, which includes a thin film transistor in which source anddrain electrode layers are in contact with a semiconductor layer, willbe illustrated in FIGS. 31A and 31B.

FIG. 31A is a cross-sectional view of a semiconductor device in which athin film transistor and a common connection portion (a pad portion) aremanufactured over the same substrate. A thin film transistor 171illustrated in FIG. 31A is an inverted staggered thin film transistorand is an example of a thin film transistor in which the source anddrain electrode layers 105 a and 105 b are provided in contact with thesemiconductor layer 103.

In the thin film transistor 171, a contact region between thesemiconductor layer 103 and the source and drain electrode layers 105 aand 105 b is preferably modified by plasma treatment. In thisembodiment, before forming a conductive film serving as the source anddrain electrode layers, an oxide semiconductor layer (an IGZOsemiconductor layer in this embodiment) is subjected to plasmatreatment.

The plasma treatment is performed using argon gas, hydrogen gas, or amixture gas of argon and hydrogen. Oxygen gas may be added to thesegases. Instead of argon gas, other rare gases may be used.

In this embodiment, the source and drain electrode layers 105 a and 105b are made of a titanium film, and subjected to wet etching using anammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2). In this etching step, an exposed regionof the semiconductor layer that is an IGZO semiconductor layer is partlyetched to be the semiconductor layer 103. Thus, a channel region of thesemiconductor layer 103 between the source and drain electrode layers105 a and 105 b has a small thickness.

A conductive layer is formed in contact with the semiconductor layer 103modified by the plasma treatment, thereby forming the source and drainelectrode layers 105 a and 105 b. Accordingly, the contact resistancebetween the semiconductor layer 103 and the source and drain electrodelayers 105 a and 105 b can be reduced.

Through the above process, a highly reliable display device as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 8

In this embodiment, an example of a display device which is one exampleof a semiconductor device of the present invention will be described. Inthe display device, at least a part of a driver circuit and a thin filmtransistor to be disposed in a pixel portion are formed over onesubstrate.

The thin film transistor to be disposed in the pixel portion is formedaccording to Embodiment 6 or 7. Further, the thin film transistordescribed in Embodiment 6 or 7 is an n-channel TFT, and thus a part of adriver circuit that can include an n-channel TFT among driver circuitsis formed over the same substrate as the thin film transistor in thepixel portion.

FIG. 13A illustrates an example of a block diagram of an active matrixliquid crystal display device which is an example of a semiconductordevice of the present invention. The display device illustrated in FIG.13A includes, over a substrate 5300, a pixel portion 5301 including aplurality of pixels each provided with a display element; a scan linedriver circuit 5302 for selecting a pixel; and a signal line drivercircuit 5303 for controlling a video signal input to the selected pixel.

The pixel portion 5301 is connected to the signal line driver circuit5303 by a plurality of signal lines S1 to Sm (not illustrated) thatextend in a column direction from the signal line driver circuit 5303,and to the scan line driver circuit 5302 by a plurality of scan lines G1to Gn (not illustrated) that extend in a row direction from the scanline driver circuit 5302. The pixel portion 5301 includes a plurality ofpixels (not illustrated) arranged in a matrix so as to correspond to thesignal lines S1 to Sm and the scan lines G1 to Gn. Each pixel isconnected to a signal line Sj (one of the signal lines S1 to Sm) and ascan line Gj (one of the scan lines G1 to Gn).

The thin film transistor described in Embodiment 6 or 7 is an n-channelTFT, and a signal line driver circuit including an n-channel TFT will bedescribed with reference to FIG. 14.

The signal line driver circuit illustrated in FIG. 14 includes a driverIC 5601, switch groups 5602-1 to 5602-M, a first wiring 5611, a secondwiring 5612, a third wiring 5613, and wirings 5621-1 to 5621-M. Each ofthe switch groups 5602-1 to 5602-M includes a first thin film transistor5603 a, a second thin film transistor 5603 b, and a third thin filmtransistor 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621-1 to 5621-M.Each of the switch groups 5602-1 to 5602-M is connected to the firstwiring 5611, the second wiring 5612, and the third wiring 5613, and theswitch groups 5602-1 to 5602-M are connected to the wirings 5621-1 to5621-M, respectively. Each of the wirings 5621-1 to 5621-M is connectedto three signal lines via the first thin film transistor 5603 a, thesecond thin film transistor 5603 b, and the third thin film transistor5603 c. For example, the wiring 5621-J of the J-th column (one of thewirings 5621-1 to 5621-M) is connected to a signal line Sj−1, a signalline Sj, and a signal line Sj+1 via the first thin film transistor 5603a, the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c which are included in the switch group 5602-J.

A signal is input to each of the first wiring 5611, the second wiring5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed over a single crystalsubstrate. Further, the switch groups 5602-1 to 5602-M are preferablyformed over the same substrate as the pixel portion. Therefore, thedriver IC 5601 is preferably connected to the switch groups 5602-1 to5602-M through an FPC or the like.

Next, operation of the signal line driver circuit illustrated in FIG. 14will be described with reference to a timing chart of FIG. 15. Thetiming chart of FIG. 15 illustrates a case where the scan line Gi of thei-th row is selected. A selection period of the scan line Gi of the i-throw is divided into a first sub-selection period T1, a secondsub-selection period T2, and a third sub-selection period T3. Inaddition, the signal line driver circuit in FIG. 14 operates in a mannersimilar to that of FIG. 15 even when a scan line of another row isselected.

Note that the timing chart of FIG. 15 illustrates a case where thewiring 5621-J of the J-th column is connected to the signal line Sj−1,the signal line Sj, and the signal line Sj+1 via the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c.

The timing chart of FIG. 15 shows the timing at which the scan line Giof the i-th row is selected, timing 5703 a at which the first thin filmtransistor 5603 a is turned on/off, timing 5703 b at which the secondthin film transistor 5603 b is turned on/off, timing 5703 c at which thethird thin film transistor 5603 c is turned on/off, and a signal 5721-Jinput to the wiring 5621-J of the J-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621-1 to 5621-M. For example, a video signal inputto the wiring 5621-J in the first sub-selection period T1 is input tothe signal line Sj−1, a video signal input to the wiring 5621-J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621-J in the third sub-selectionperiod T3 is input to the signal line Sj+1. The video signals input tothe wiring 5621-J in the first sub-selection period T1, the secondsub-selection period T2, and the third sub-selection period T3 aredenoted by Data-j−1, Data-j, and Data j+1, respectively.

As illustrated in FIG. 15, in the first sub-selection period T1, thefirst thin film transistor 5603 a is turned on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are turnedoff. At this time, Data-j−1 input to the wiring 5621-J is input to thesignal line Sj−1 via the first thin film transistor 5603 a. In thesecond sub-selection period T2, the second thin film transistor 5603 bis turned on, and the first thin film transistor 5603 a and the thirdthin film transistor 5603 c are turned off. At this time, Data-j inputto the wiring 5621-J is input to the signal line Sj via the second thinfilm transistor 5603 b. In the third sub-selection period T3, the thirdthin film transistor 5603 c is turned on, and the first thin filmtransistor 5603 a and the second thin film transistor 5603 b are turnedoff. At this time, Data j+1 input to the wiring 5621-J is input to thesignal line Sj+1 via the third thin film transistor 5603 c.

As described above, in the signal line driver circuit of FIG. 14, bydividing one gate selection period into three, video signals can beinput to three signal lines from one wiring 5621 in one gate selectionperiod. Therefore, in the signal line driver circuit of FIG. 14, thenumber of connections between the substrate provided with the driver IC5601 and the substrate provided with the pixel portion can be reduced toapproximately one third of the number of signal lines. When the numberof connections is reduced to approximately one third of the number ofthe signal lines, the reliability, yield, and the like of the signalline driver circuit of FIG. 14 can be improved.

Note that there are no particular limitations on the arrangement,number, driving method, and the like of the thin film transistors, aslong as one gate selection period is divided into a plurality ofsub-selection periods and video signals are input to a plurality ofsignal lines from one wiring in the respective sub-selection periods asillustrated in FIG. 14.

For example, when video signals are input to three or more signal linesfrom one wiring in three or more sub-selection periods, it is onlynecessary to add a thin film transistor and a wiring for controlling thethin film transistor. Note that when one gate selection period isdivided into four or more sub-selection periods, one sub-selectionperiod becomes shorter. Therefore, one gate selection period ispreferably divided into two or three sub-selection periods.

As another example, one selection period may be divided into a prechargeperiod Tp, the first sub-selection period T1, the second sub-selectionperiod T2, and the third sub-selection period T3 as illustrated in atiming chart of FIG. 16. The timing chart of FIG. 16 illustrates thetiming at which the scan line Gi of the i-th row is selected, timing5803 a at which the first thin film transistor 5603 a is turned on/off,timing 5803 b at which the second thin film transistor 5603 b is turnedon/off, timing 5803 c at which the third thin film transistor 5603 c isturned on/off, and a signal 5821-J input to the wiring 5621-J of theJ-th column. As illustrated in FIG. 16, the first thin film transistor5603 a, the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c are tuned on in the precharge period Tp. At this time,precharge voltage Vp input to the wiring 5621-J is input to each of thesignal line Sj−1, the signal line Sj, and the signal line Sj+1 via thefirst thin film transistor 5603 a, the second thin film transistor 5603b, and the third thin film transistor 5603 c. In the first sub-selectionperiod T1, the first thin film transistor 5603 a is turned on, and thesecond thin film transistor 5603 b and the third thin film transistor5603 c are turned off. At this time, Data-j−1 input to the wiring 5621-Jis input to the signal line Sj−1 via the first thin film transistor 5603a. In the second sub-selection period T2, the second thin filmtransistor 5603 b is turned on, and the first thin film transistor 5603a and the third thin film transistor 5603 c are turned off. At thistime, Data-j input to the wiring 5621-J is input to the signal line Sjvia the second thin film transistor 5603 b. In the third sub-selectionperiod T3, the third thin film transistor 5603 c is turned on, and thefirst thin film transistor 5603 a and the second thin film transistor5603 b are turned off. At this time, Data j+1 input to the wiring 5621-Jis input to the signal line Sj+1 via the third thin film transistor 5603c.

As described above, in the signal line driver circuit of FIG. 14 towhich the timing chart of FIG. 16 is applied, the video signal can bewritten to the pixel at high speed because the signal line can beprecharged by providing a precharge selection period before asub-selection period. Note that portions of FIG. 16 which are similar tothose of FIG. 15 are denoted by common reference numerals and detaileddescription of like portions and portions having a similar function isomitted.

Further, a structure of a scan line driver circuit is described. Thescan line driver circuit includes a shift register and a buffer.Additionally, the scan line driver circuit may include a level shifterin some cases. In the scan line driver circuit, when the clock signal(CLK) and the start pulse signal (SP) are input to the shift register, aselection signal is generated. The generated selection signal isbuffered and amplified by the buffer, and the resulting signal issupplied to a corresponding scan line. Gate electrodes of transistors inpixels of one line are connected to the scan line. Since the transistorsin the pixels of one line have to be turned on all at once, a bufferwhich can supply a large current is used.

One mode of a shift register used for a part of the scan line drivercircuit will be described with reference to FIG. 17 and FIG. 18.

FIG. 17 illustrates a circuit configuration of the shift register. Theshift register illustrated in FIG. 17 includes a plurality of flip-flops(flip-flops 5701-1 to 5701-n). The shift register operates with theinput of a first clock signal, a second clock signal, a start pulsesignal, and a reset signal.

The connection relationship of the shift register of FIG. 17 will bedescribed. In the i-th stage flip-flop 5701-i (one of the flip-flops5701-1 to 5701-n) in the shift register of FIG. 17, a first wiring 5501illustrated in FIG. 18 is connected to a seventh wiring 5717-1−1; asecond wiring 5502 illustrated in FIG. 18 is connected to a seventhwiring 5717-i+1; a third wiring 5503 illustrated in FIG. 18 is connectedto a seventh wiring 5717-i; and a sixth wiring 5506 illustrated in FIG.18 is connected to a fifth wiring 5715.

Further, a fourth wiring 5504 illustrated in FIG. 18 is connected to asecond wiring 5712 in flip-flops of odd-numbered stages, and isconnected to a third wiring 5713 in flip-flops of even-numbered stages.A fifth wiring 5505 illustrated in FIG. 18 is connected to a fourthwiring 5714.

Note that the first wiring 5501 of the first stage flip-flop 5701-1illustrated in FIG. 18 is connected to a first wiring 5711. Moreover,the second wiring 5502 of the n-th stage flip-flop 5701-n illustrated inFIG. 18 is connected to a sixth wiring 5716.

Note that the first wiring 5711, the second wiring 5712, the thirdwiring 5713, and the sixth wiring 5716 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fourth wiring 5714 and the fifth wiring5715 may be referred to as a first power supply line and a second powersupply line, respectively.

Next, FIG. 18 illustrates details of the flip-flop illustrated in FIG.17. A flip-flop illustrated in FIG. 18 includes a first thin filmtransistor 5571, a second thin film transistor 5572, a third thin filmtransistor 5573, a fourth thin film transistor 5574, a fifth thin filmtransistor 5575, a sixth thin film transistor 5576, a seventh thin filmtransistor 5577, and an eighth thin film transistor 5578. Each of thefirst thin film transistor 5571, the second thin film transistor 5572,the third thin film transistor 5573, the fourth thin film transistor5574, the fifth thin film transistor 5575, the sixth thin filmtransistor 5576, the seventh thin film transistor 5577, and the eighththin film transistor 5578 is an n-channel transistor and is turned onwhen the gate-source voltage (V_(gs)) exceeds the threshold voltage(V_(th)).

Next, connection structures of the flip-flop illustrated in FIG. 18 willbe described below.

A first electrode (one of a source electrode and a drain electrode) ofthe first thin film transistor 5571 is connected to the fourth wiring5504. A second electrode (the other of the source electrode and thedrain electrode) of the first thin film transistor 5571 is connected tothe third wiring 5503.

A first electrode of the second thin film transistor 5572 is connectedto the sixth wiring 5506. A second electrode of the second thin filmtransistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected tothe fifth wiring 5505 and a second electrode of the third thin filmtransistor is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the third thin film transistor 5573is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connectedto the sixth wiring 5506. A second electrode of the fourth thin filmtransistor 5574 is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the fourth thin film transistor5574 is connected to a gate electrode of the first thin film transistor5571.

A first electrode of the fifth thin film transistor 5575 is connected tothe fifth wiring 5505. A second electrode of the fifth thin filmtransistor 5575 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the fifth thin film transistor5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected tothe sixth wiring 5506. A second electrode of the sixth thin filmtransistor 5576 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the sixth thin film transistor5576 is connected to the gate electrode of the second thin filmtransistor 5572.

A first electrode of the seventh thin film transistor 5577 is connectedto the sixth wiring 5506. A second electrode of the seventh thin filmtransistor 5577 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the seventh thin filmtransistor 5577 is connected to the second wiring 5502. A firstelectrode of the eighth thin film transistor 5578 is connected to thesixth wiring 5506. A second electrode of the eighth thin film transistor5578 is connected to the gate electrode of the second thin filmtransistor 5572. A gate electrode of the eighth thin film transistor5578 is connected to the first wiring 5501.

Note that the points at which the gate electrode of the first thin filmtransistor 5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor 5575, thesecond electrode of the sixth thin film transistor 5576, and the secondelectrode of the seventh thin film transistor 5577 are connected areeach referred to as a node 5543. The points at which the gate electrodeof the second thin film transistor 5572, the second electrode of thethird thin film transistor 5573, the second electrode of the fourth thinfilm transistor 5574, the gate electrode of the sixth thin filmtransistor 5576, and the second electrode of the eighth thin filmtransistor 5578 are connected are each referred to as a node 5544.

Note that the first wiring 5501, the second wiring 5502, the thirdwiring 5503, and the fourth wiring 5504 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fifth wiring 5505 and the sixth wiring5506 may be referred to as a first power supply line and a second powersupply line, respectively.

In addition, the signal line driver circuit and the scan line drivercircuit can be formed using only n-channel TFTs described in Embodiment6. Since the n-channel TFT described in Embodiment 6 has a highmobility, the driving frequency of a driver circuit can be increased. Inaddition, in the n-channel TFT described in Embodiment 6, sinceparasitic capacitance is reduced by the source or drain region that isan oxide-deficient oxide semiconductor layer containing indium, gallium,and zinc, high frequency characteristics (referred to as Fcharacteristics) can be obtained. For example, a scan line drivercircuit using the n-channel TFT described in Embodiment 6 can operate athigh speed, and thus a frame frequency can be increased and insertion ofblack images can be realized.

In addition, when the channel width of the transistor in the scan linedriver circuit is increased or a plurality of scan line driver circuitsare provided, higher frame frequency can be realized. When a pluralityof scan line driver circuits are provided, a scan line driver circuitfor driving even-numbered scan lines is provided on one side and a scanline driver circuit for driving odd-numbered scan lines is provided onthe opposite side; thus, an increase in frame frequency can be realized.

Further, when an active matrix light-emitting display device which is anexample of a semiconductor device of the present invention ismanufactured, a plurality of thin film transistors are arranged in atleast one pixel, and thus a plurality of scan line driver circuits arepreferably arranged. FIG. 13B is an example of a block diagram of anactive matrix light-emitting display device.

The light-emitting display device illustrated in FIG. 13B includes, overa substrate 5400, a pixel portion 5401 having a plurality of pixels eachprovided with a display element, a first scan line driver circuit 5402and a second scan line driver circuit 5404 for selecting a pixel, and asignal line driver circuit 5403 for controlling input of a video signalto the selected pixel.

When the video signal input to a pixel of the light-emitting displaydevice illustrated in FIG. 13B is a digital signal, a pixel emits lightor does not emit light by switching a transistor on/off. Thus, grayscalecan be displayed using an area grayscale method or a time grayscalemethod. An area grayscale method refers to a driving method in which onepixel is divided into a plurality of subpixels and the respectivesubpixels are driven independently based on video signals so thatgrayscale is displayed. Further, a time grayscale method refers to adriving method in which a period during which a pixel emits light iscontrolled so that grayscale is displayed.

Since the response time of a light-emitting element is higher than thatof a liquid crystal element or the like, the light-emitting element ismore suitable for a time grayscale method than the liquid crystalelement. Specifically, in the case of displaying with a time grayscalemethod, one frame period is divided into a plurality of subframeperiods. Then, in accordance with video signals, the light-emittingelement in the pixel is brought into a light-emitting state or anon-light-emitting state in each subframe period. By dividing one frameperiod into a plurality of subframe periods, the total length of time,in which a pixel actually emits light in one frame period, can becontrolled by video signals so that grayscale can be displayed.

In the light-emitting display device illustrated in FIG. 13B, in a casewhere two TFTs of a switching TFT and a current control TFT are arrangedin one pixel, the first scan line driver circuit 5402 generates a signalwhich is input to a first scan line serving as a gate wiring of theswitching TFT, and the second scan line driver circuit 5404 generates asignal which is input to a second scan line serving as a gate wiring ofthe current control TFT; however, one scan line driver circuit maygenerate both the signal which is input to the first scan line and thesignal which is input to the second scan line. In addition, for example,there is a possibility that a plurality of the first scan lines used forcontrolling the operation of the switching element are provided in eachpixel, depending on the number of transistors included in the switchingelement. In that case, one scan line driver circuit may generate allsignals that are input to the plurality of first scan lines, or aplurality of scan line driver circuits may generate signals that areinput to the plurality of first scan lines.

Also in the light-emitting display device, a part of a driver circuitthat can include n-channel TFTs among driver circuits can be formed overthe same substrate as the thin film transistors of the pixel portion.Alternatively, the signal line driver circuit and the scan line drivercircuit can be formed using only the n-channel TFTs described inEmbodiment 6 or 7.

Moreover, the above-described driver circuit can be used for electronicpaper that drives electronic ink using an element electrically connectedto a switching element, without being limited to applications to aliquid crystal display device or a light-emitting display device. Theelectronic paper is also referred to as an electrophoretic displaydevice (electrophoretic display) and is advantageous in that it has thesame level of readability as plain paper, it has lower power consumptionthan other display devices, and it can be made thin and lightweight.

Electrophoretic displays can have various modes. Electrophoreticdisplays contain a plurality of microcapsules dispersed in a solvent ora solute, each microcapsule containing first particles which arepositively charged and second particles which are negatively charged. Byapplying an electric field to the microcapsules, the particles in themicrocapsules move in opposite directions to each other and only thecolor of the particles gathering on one side is displayed. Note that thefirst particles and the second particles each contain pigment and do notmove without an electric field. Moreover, the first particles and thesecond particles have different colors (which may be colorless).

Thus, an electrophoretic display is a display that utilizes a so-calleddielectrophoretic effect by which a substance having a high dielectricconstant moves to a high-electric field region. An electrophoreticdisplay device does not need to use a polarizer or a counter substrate,which is required in a liquid crystal display device, and both thethickness and weight of the electrophoretic display device can bereduced to a half of those of a liquid crystal display device.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

In addition, if a plurality of the above microcapsules are arranged asappropriate over an active matrix substrate so as to be interposedbetween two electrodes, an active matrix display device can becompleted, and display can be performed by application of an electricfield to the microcapsules. For example, the active matrix substrateobtained by the thin film transistors described in Embodiment 6 or 7 canbe used.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

Through the above process, a highly reliable display device as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 9

When a thin film transistor of one embodiment of the present inventionis manufactured and used for a pixel portion and further for a drivercircuit, a semiconductor device having a display function (also referredto as a display device) can be manufactured. Furthermore, when part orwhole of a driver circuit using a thin film transistor of one embodimentof the present invention is formed over the same substrate as a pixelportion, a system-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. Light-emitting elementsinclude, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically include an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as an electronic ink, can be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel. An embodiment of the presentinvention also relates to an element substrate, which corresponds to onemode before the display element is completed in a manufacturing processof the display device, and the element substrate is provided with meansfor supplying current to the display element in each of a plurality ofpixels. Specifically, the element substrate may be in a state after onlya pixel electrode of the display element is formed, a state after aconductive film to be a pixel electrode is formed and before theconductive film is etched to form the pixel electrode, or any of otherstates.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as aflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP) is attached; a module having a TAB tape ora TCP at the tip of which a printed wiring board is provided; and amodule in which an integrated circuit (IC) is directly mounted on adisplay element by chip on glass (COG).

In this embodiment, the appearance and a cross section of a liquidcrystal display panel, which is one embodiment of the semiconductordevice of the present invention, will be described with reference toFIGS. 21A and 21B. FIGS. 21A1 and 21A2 are top views of a panel in whichhighly reliable thin film transistors 4010 and 4011 shown in Embodiment6, each of which includes an oxygen-excess oxide semiconductor layer asa channel formation region and an oxygen-deficient oxide semiconductorlayer as source and drain regions, and a liquid crystal element 4013 aresealed between a first substrate 4001 and a second substrate 4006 with asealant 4005. FIG. 21B is a cross-sectional view taken along line M-N ofFIGS. 21A1 and 21A2.

The sealant 4005 is provided to surround a pixel portion 4002 and ascanning line driver circuit 4004 that are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scanning line driver circuit 4004. Therefore, thepixel portion 4002 and the scanning line driver circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region different from the region surrounded bythe sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofa driver circuit which is separately formed, and COG, wire bonding, TAB,or the like can be used. FIG. 21A1 illustrates an example of mountingthe signal line driver circuit 4003 by COG, and FIG. 21A2 illustrates anexample of mounting the signal line driver circuit 4003 by TAB.

The pixel portion 4002 and the scanning line driver circuit 4004provided over the first substrate 4001 each include a plurality of thinfilm transistors. FIG. 21B illustrates the thin film transistor 4010included in the pixel portion 4002 and the thin film transistor 4011included in the scanning line driver circuit 4004. Insulating layers4020 and 4021 are provided over the thin film transistors 4010 and 4011.

As the thin film transistors 4010 and 4011, the highly reliable thinfilm transistor shown in Embodiment 6, which includes an oxygen-excessoxide semiconductor layer as a channel formation region and anoxygen-deficient oxide semiconductor layer as source and drain regionsmay be employed. Alternatively, the thin film transistor shown inEmbodiment 7 may be employed as the thin film transistors 4010 and 4011.In this embodiment, the thin film transistors 4010 and 4011 aren-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with one another corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033, respectively, each of which functions as an alignment film. Theliquid crystal layer 4008 is sandwiched between the pixel electrodelayer 4030 and the counter electrode layer 4031 with the insulatinglayers 4032 and 4033 interposed therebetween.

Note that the first substrate 4001 and the second substrate 4006 can bemade of glass, metal (typically, stainless steel), ceramic, or plastic.As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinylfluoride (PVF) film, a polyester film, or an acrylic resin film can beused. Alternatively, a sheet with a structure in which an aluminum foilis sandwiched between PVF films or polyester films can be used.

Reference numeral 4035 denotes a columnar spacer obtained by selectivelyetching an insulating film and is provided to control the distancebetween the pixel electrode layer 4030 and the counter electrode layer4031 (a cell gap). Alternatively, a spherical spacer may be used. Thecounter electrode layer 4031 is electrically connected to a commonpotential line provided over the same substrate as the thin filmtransistor 4010. With the use of the common connection portion show inany of Embodiments 1 to 3, the counter electrode layer 4031 iselectrically connected to the common potential line through conductiveparticles provided between the pair of substrates. Note that theconductive particles are contained in the sealant 4005.

Alternatively, a liquid crystal showing a blue phase for which analignment film is unnecessary may be used. A blue phase is one of theliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperature, a liquid crystal compositioncontaining a chiral agent at 5 wt % or more is used for the liquidcrystal layer 4008 in order to improve the temperature range. The liquidcrystal composition which includes a liquid crystal showing a blue phaseand a chiral agent has a small response time of 10 μs to 100 μs, hasoptical isotropy, which makes the alignment process unneeded, and has asmall viewing angle dependence

Although an example of a transmissive liquid crystal display device isshown in this embodiment, an embodiment of the present invention canalso be applied to a reflective liquid crystal display device or atransflective liquid crystal display device.

In this embodiment, an example of the liquid crystal display device isshown in which a polarizing plate is provided on the outer surface ofthe substrate (on the viewer side) and a coloring layer and an electrodelayer used for a display element are provided on the inner surface ofthe substrate in this order; however, the polarizing plate may beprovided on the inner surface of the substrate. The stacked structure ofthe polarizing plate and the coloring layer is not limited to that shownin this embodiment and may be set as appropriate depending on materialsof the polarizing plate and the coloring layer or conditions ofmanufacturing steps. Furthermore, a light-blocking film serving as ablack matrix may be provided.

In this embodiment, in order to reduce the surface roughness of the thinfilm transistor and to improve the reliability of the thin filmtransistor, the thin film transistor obtained by Embodiment 6 is coveredwith the insulating layers (the insulating layer 4020 and the insulatinglayer 4021) serving as a protective film or a planarizing insulatingfilm. Note that the protective film is provided to prevent entry ofimpurities floating in the air, such as an organic substance, a metalsubstance, or moisture, and is preferably a dense film. The protectivefilm may be formed by sputtering to be a single-layer film or amulti-layer film of a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum nitride film, an aluminum oxynitride film, and/or analuminum nitride oxide film. Although this embodiment shows an exampleof forming the protective film by sputtering, the present invention isnot limited to this method and a variety of methods may be employed.

In this embodiment, the insulating layer having a multi-layer structureis formed as the protective film. As a first layer of the insulatinglayer 4020, a silicon oxide film is formed by sputtering. The use of thesilicon oxide film as the protective film has an effect of preventing ahillock of an aluminum film used for the source and drain electrodelayers.

The insulating layer is also formed as a second layer of the protectivefilm. In this embodiment, as a second layer of the insulating layer4020, a silicon nitride film is formed by sputtering. The use of thesilicon nitride film as the protective film can prevent mobile ions suchas sodium ions from entering a semiconductor region, thereby suppressingvariations in electrical properties of the TFT.

After the protective film is formed, the IGZO semiconductor layer may beannealed (at 300° C. to 400° C.).

The insulating layer 4021 is formed as the planarizing insulating film.For the insulating layer 4021, an organic material having heatresistance, such as polyimide, acrylic, polyimide, benzocyclobutene,polyamide, or epoxy, can be used. Other than such organic materials, itis also possible to use a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. A siloxane-based resin mayinclude as a substituent at least one of fluorine, an alkyl group, andan aryl group, as well as hydrogen. Note that the insulating layer 4021may be formed by stacking a plurality of insulating films formed ofthese materials.

Note that a siloxane-based resin is a resin formed from a siloxanematerial as a starting material and having the bond of Si—O—Si. Thesiloxane-based resin may include as a substituent at least one offluorine, an alkyl group, and aromatic hydrocarbon, as well as hydrogen.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on the material, by sputtering, SOG, spin coating, dipping,spray coating, droplet discharging (e.g., ink-jet, screen printing, oroffset printing), doctor knife, roll coater, curtain coater, knifecoater, or the like. In the case where the insulating layer 4021 isformed using a material solution, the IGZO semiconductor layer may beannealed (at 300° C. to 400° C.) at the same time of a baking step. Thebaking step of the insulating layer 4021 also serves as the annealingstep of the IGZO semiconductor layer, whereby a semiconductor device canbe manufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe made of a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodemade of the conductive composition preferably has a sheet resistance of10000 ohms per square or less and a transmittance of 70% or more at awavelength of 550 nm. Furthermore, the resistivity of the conductivehigh molecule contained in the conductive composition is preferably 0.1Ω·cm or less.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. For example, it is possible to usepolyaniline or a derivative thereof, polypyrrole or a derivativethereof, polythiophene or a derivative thereof, or a copolymer of two ormore kinds of them.

In addition, a variety of signals and a potential are supplied to thesignal line driver circuit 4003 that is formed separately, and thescanning line driver circuit 4004 or the pixel portion 4002 from an FPC4018.

In this embodiment, a connecting terminal electrode 4015 is formed usingthe same conductive film as that of the pixel electrode layer 4030included in the liquid crystal element 4013, and a terminal electrode4016 is formed using the same conductive film as that of source anddrain electrode layers of the thin film transistors 4010 and 4011.

The connecting terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 21A and 21B illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, this embodiment is not limited to thisstructure. The scanning line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scanning line driver circuit may be separately formed and thenmounted.

FIG. 22 illustrates an example of a liquid crystal display module whichis formed as a semiconductor device by using a TFT substrate 2600manufactured by the present invention.

FIG. 22 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are bonded toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, respective coloring layerscorresponding to colors of red, green, and blue are provided forrespective pixels. Polarizing plates 2606 and 2607 and a diffusion plate2613 are provided outside the TFT substrate 2600 and the countersubstrate 2601. A light source includes a cold cathode tube 2610 and areflective plate 2611. A circuit board 2612 is connected to a wiringcircuit portion 2608 of the TFT substrate 2600 through a flexible wiringboard 2609 and includes an external circuit such as a control circuit ora power source circuit. The polarizing plate and the liquid crystallayer may be stacked with a retardation plate interposed therebetween.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above process, a highly reliable liquid crystal displaypanel as a semiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 10

In this embodiment, an example of electronic paper will be described asa semiconductor device of one embodiment of the present invention.

FIG. 12 illustrates active matrix electronic paper as an example of asemiconductor device to which the present invention is applied. A thinfilm transistor 581 used for the semiconductor device can bemanufactured in a manner similar to the thin film transistor shown inEmbodiment 6 and is a highly reliable thin film transistor including anoxygen-excess oxide semiconductor layer as a channel formation regionand an oxygen-deficient oxide semiconductor layer as source and drainregions. The thin film transistor shown in Embodiment 7 can also be usedas the thin film transistor 581 of this embodiment.

The electronic paper in FIG. 12 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control the orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 which is sealed between a substrate 580 anda substrate 596 is a bottom-gate thin film transistor, and a source ordrain electrode layer is in contact with a first electrode layer 587through an opening formed in an insulating layer 585, whereby the thinfilm transistor 581 is electrically connected to the first electrodelayer 587. Between the first electrode layer 587 and a second electrodelayer 588, spherical particles 589 each having a black region 590 a, awhite region 590 b, and a cavity 594 around the regions which is filledwith liquid are provided. A space around the spherical particles 589 isfilled with a filler 595 such as a resin (see FIG. 12). In thisembodiment, the first electrode layer 587 corresponds to the pixelelectrode and the second electrode layer 588 corresponds to the commonelectrode. The second electrode layer 588 is electrically connected to acommon potential line provided over the same substrate as the thin filmtransistor 581. With the use of the common connection portion show inany of Embodiments 1 to 3, the second electrode layer 588 iselectrically connected to the common potential line through conductiveparticles provided between the pair of substrates.

Instead of the twisting ball, an electrophoretic element can also beused. A microcapsule having a diameter of about 10 μm to 200 μm in whichtransparent liquid, positively-charged white microparticles, andnegatively-charged black microparticles are encapsulated, is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied betweenthe first electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element and isgenerally called electronic paper. The electrophoretic display elementhas higher reflectance than a liquid crystal display element, and thus,an auxiliary light is unnecessary, power consumption is low, and adisplay portion can be recognized in a dim place. In addition, even whenpower is not supplied to the display portion, an image which has beendisplayed once can be maintained. Accordingly, a displayed image can bestored even if a semiconductor device having a display function (whichmay be referred to simply as a display device or a semiconductor deviceprovided with a display device) is distanced from an electric wavesource.

Through the above process, a highly reliable electronic paper as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures of the common connection portion described in any one ofembodiments 1 to 5.

Embodiment 11

In this embodiment, an example of a light-emitting display device willbe described as a semiconductor device of one embodiment of the presentinvention. As a display element included in a display device, alight-emitting element utilizing electroluminescence is described here.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. Then, the carriers (electrons and holes) arerecombined, so that the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element.

FIG. 19 illustrates an example of a pixel structure as an example of asemiconductor device of the present invention, which can be driven by adigital time grayscale method.

The structure and operation of a pixel which can be driven by a digitaltime grayscale method will be described. An example is shown here inwhich one pixel includes two n-channel transistors using an oxidesemiconductor layer (an IGZO semiconductor layer) in a channel formationregion.

A pixel 6400 includes a switching transistor 6401, a driving transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driving transistor 6402. The gate of the driving transistor 6402 isconnected to a power supply line 6407 through the capacitor 6403, afirst electrode of the driving transistor 6402 is connected to the powersupply line 6407, and a second electrode of the driving transistor 6402is connected to a first electrode (pixel electrode) of thelight-emitting element 6404. A second electrode of the light-emittingelement 6404 corresponds to a common electrode 6408. The commonelectrode 6408 is electrically connected to a common potential lineprovided over the same substrate, and the structure illustrated in FIG.1A, FIG. 2A, or FIG. 3A may be obtained using the connection portion asa common connection portion.

Note that the second electrode (the common electrode 6408) of thelight-emitting element 6404 is set to a low power supply potential. Thelow power supply potential is lower than a high power supply potentialwhich is supplied to the power supply line 6407. For example, GND or 0 Vmay be set as the low power supply potential. The difference between thehigh power supply potential and the low power supply potential isapplied to the light-emitting element 6404 to flow current in thelight-emitting element 6404, whereby the light-emitting element 6404emits light. Thus, each potential is set so that the difference betweenthe high power supply potential and the low power supply potential isequal to or higher than a forward threshold voltage.

When the gate capacitance of the driving transistor 6402 is used as asubstitute for the capacitor 6403, the capacitor 6403 can be omitted.The gate capacitance of the driving transistor 6402 may be formedbetween a channel region and a gate electrode.

In the case of using a voltage-input voltage driving method, a videosignal is input to the gate of the driving transistor 6402 to make thedriving transistor 6402 completely turn on or off. That is, the drivingtransistor 6402 operates in a linear region, and thus, a voltage higherthan the voltage of the power supply line 6407 is applied to the gate ofthe driving transistor 6402. Note that a voltage higher than or equal to(power supply line voltage+V_(th) of the driving transistor 6402) isapplied to the signal line 6405.

In the case of using an analog grayscale method instead of the digitaltime grayscale method, the same pixel structure as in FIG. 19 can beemployed by inputting signals in a different way.

In the case of using the analog grayscale method, a voltage higher thanor equal to (forward voltage of the light-emitting element 6404+V_(th)of the driving transistor 6402) is applied to the gate of the drivingtransistor 6402. The forward voltage of the light-emitting element 6404refers to a voltage to obtain a desired luminance, and includes at leasta forward threshold voltage. By inputting a video signal to allow thedriving transistor 6402 to operate in a saturation region, current canflow in the light-emitting element 6404. In order to allow the drivingtransistor 6402 to operate in the saturation region, the potential ofthe power supply line 6407 is higher than a gate potential of thedriving transistor 6402. Since the video signal is an analog signal, acurrent in accordance with the video signal flows in the light-emittingelement 6404, and the analog grayscale method can be performed.

Note that the pixel structure is not limited to that illustrated in FIG.19. For example, the pixel in FIG. 26 can further include a switch, aresistor, a capacitor, a transistor, a logic circuit, or the like.

Next, structures of the light-emitting element will be described withreference to FIGS. 20A to 20C. A cross-sectional structure of a pixelwill be described by taking an n-channel driving TFT as an example.Driving TFTs 7001, 7011, and 7021 used for semiconductor devicesillustrated in FIGS. 20A to 20C can be manufactured in a manner similarto the thin film transistor described in Embodiment 6 and are highlyreliable thin film transistors each including an oxygen-excess oxidesemiconductor layer as a channel formation region and anoxygen-deficient oxide semiconductor layer as source and drain regions.Alternatively, the thin film transistor described in Embodiment 7 can beemployed as the driving TFTs 7001, 7011, and 7021.

In order to extract light emitted from the light-emitting element, atleast one of the anode and the cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure inwhich light is extracted through the surface opposite to the substrate;a bottom emission structure in which light is extracted through thesurface on the substrate side; or a dual emission structure in whichlight is extracted through the surface opposite to the substrate and thesurface on the substrate side. The pixel structure of an embodiment ofthe present invention can be applied to a light-emitting element havingany of these emission structures.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 20A.

FIG. 20A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is of an n-type and light is emitted from alight-emitting element 7002 to an anode 7005 side. In FIG. 20A, acathode 7003 of the light-emitting element 7002 is electricallyconnected to the driving TFT 7001, and a light-emitting layer 7004 andthe anode 7005 are stacked in this order over the cathode 7003. Thecathode 7003 can be made of a variety of conductive materials as long asthey have a low work function and reflect light. For example, Ca, Al,CaF, MgAg, AlLi, or the like is preferably used. The light-emittinglayer 7004 may be formed using a single layer or a plurality of layersstacked. When the light-emitting layer 7004 is formed using a pluralityof layers, the light-emitting layer 7004 is formed by stacking anelectron-injecting layer, an electron-transporting layer, alight-emitting layer, a hole-transporting layer, and a hole-injectinglayer in this order over the cathode 7003. Not all of these layers needto be provided. The anode 7005 is made of a light-transmittingconductive material such as indium oxide including tungsten oxide,indium zinc oxide including tungsten oxide, indium oxide includingtitanium oxide, indium tin oxide including titanium oxide, indium tinoxide (hereinafter referred to as ITO), indium zinc oxide, or indium tinoxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where thecathode 7003 and the anode 7005 sandwich the light-emitting layer 7004.In the case of the pixel illustrated in FIG. 20A, light is emitted fromthe light-emitting element 7002 to the anode 7005 side as indicated byan arrow.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 20B. FIG. 20B is a cross-sectionalview of a pixel in the case where the driving TFT 7011 is of an n-typeand light is emitted from a light-emitting element 7012 to a cathode7013 side. In FIG. 20B, the cathode 7013 of the light-emitting element7012 is formed over a light-transmitting conductive film 7017 which iselectrically connected to the driving TFT 7011, and a light-emittinglayer 7014 and an anode 7015 are stacked in this order over the cathode7013. A light-blocking film 7016 for reflecting or blocking light may beformed to cover the anode 7015 when the anode 7015 has alight-transmitting property. For the cathode 7013, various materials canbe used, like in the case of FIG. 20A, as long as they are conductivematerials having a low work function. Note that the cathode 7013 isformed to have a thickness that can transmit light (preferably,approximately 5 nm to 30 nm). For example, an aluminum film with athickness of 20 nm can be used as the cathode 7013. Similarly to thecase of FIG. 20A, the light-emitting layer 7014 may be formed usingeither a single layer or a plurality of layers stacked. The anode 7015is not required to transmit light, but can be made of alight-transmitting conductive material like in the case of FIG. 20A. Asthe light-blocking film 7016, a metal which reflects light can be usedfor example; however, it is not limited to a metal film. For example, aresin to which black pigments are added can also be used.

The light-emitting element 7012 corresponds to a region where thecathode 7013 and the anode 7015 sandwich the light-emitting layer 7014.In the case of the pixel illustrated in FIG. 20B, light is emitted fromthe light-emitting element 7012 to the cathode 7013 side as indicated byan arrow.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 20C. In FIG. 20C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. Like in the case of FIG. 20A, thecathode 7023 can be made of a variety of conductive materials as long asthey have a low work function. Note that the cathode 7023 is formed tohave a thickness that can transmit light. For example, a film of Alhaving a thickness of 20 nm can be used as the cathode 7023. Like inFIG. 20A, the light-emitting layer 7024 may be formed using either asingle layer or a plurality of layers stacked. The anode 7025 can bemade of a light-transmitting conductive material like in the case ofFIG. 20A.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith one another. In the case of the pixel illustrated in FIG. 20C,light is emitted from the light-emitting element 7022 to both the anode7025 side and the cathode 7023 side as indicated by arrows.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can also be provided as alight-emitting element.

In this embodiment, the example is described in which a thin filmtransistor (a driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement; however, a structure may be employed in which a TFT for currentcontrol is connected between the driving TFT and the light-emittingelement.

The structure of the semiconductor device described in this embodimentis not limited to those illustrated in FIGS. 20A to 20C and can bemodified in various ways based on the spirit of techniques of thepresent invention.

Next, the appearance and a cross section of a light-emitting displaypanel, which is one embodiment of the semiconductor device of thepresent invention, will be described with reference to FIGS. 23A and23B. FIG. 23A is a top view of a panel in which a thin film transistorand a light-emitting element are sealed between a first substrate and asecond substrate with a sealant. FIG. 23B is a cross-sectional viewtaken along line H-I of FIG. 23A.

A sealant 4505 is provided to surround a pixel portion 4502, signal linedriver circuits 4503 a and 4503 b, and scanning line driver circuits4504 a and 4504 b, which are provided over a first substrate 4501. Inaddition, a second substrate 4506 is provided over the pixel portion4502, the signal line driver circuits 4503 a and 4503 b, and thescanning line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescanning line driver circuits 4504 a and 4504 b are sealed together witha filler 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a display device be thuspackaged (sealed) with a protective film (such as a bonding film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the display device isnot exposed to the outside air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scanning line driver circuits 4504 a and 4504 b formed overthe first substrate 4501 each include a plurality of thin filmtransistors, and a thin film transistor 4510 included in the pixelportion 4502 and a thin film transistor 4509 included in the signal linedriver circuit 4503 a are illustrated as an example in FIG. 23B.

As the thin film transistors 4509 and 4510, the thin film transistordescribed in Embodiment 6, which includes an oxygen-excess oxidesemiconductor layer as a channel formation region and anoxygen-deficient oxide semiconductor layer as source and drain regions,can be employed. Alternatively, the thin film transistor described inEmbodiment 7 may be used as the thin film transistors 4509 and 4510. Inthis embodiment, the thin film transistors 4509 and 4510 are n-channelthin film transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 that is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that a structure of the light-emitting element 4511 is notlimited to the stacked structure shown in this embodiment, whichincludes the first electrode layer 4517, an electroluminescent layer4512, and the second electrode layer 4513. The structure of thelight-emitting element 4511 can be changed as appropriate depending onthe direction in which light is extracted from the light-emittingelement 4511, or the like.

A partition wall 4520 is made of an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition wall 4520 be formed of a photosensitive material tohave an opening over the first electrode layer 4517 so that a sidewallof the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may be formed using a single layer ora plurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition wall 4520 in order to prevent oxygen, hydrogen, moisture,carbon dioxide, or the like from entering into the light-emittingelement 4511. As the protective film, a silicon nitride film, a siliconnitride oxide film, a DLC film, or the like can be formed.

A variety of signals and potentials are supplied to the signal linedriver circuits 4503 a and 4503 b, the scanning line driver circuits4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 a and 4518b.

In this embodiment, a connection terminal electrode 4515 is formed usingthe same conductive film as the first electrode layer 4517 include inthe light-emitting element 4511, and a terminal electrode 4516 is formedusing the same conductive film as the source and drain electrode layersincluded in the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive film 4519.

The second substrate 4506 located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used. In this embodiment, nitrogen is used for thefiller 4507.

If needed, an optical film, such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter, may be provided as appropriate on a light-emitting surfaceof the light-emitting element. Furthermore, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scanning linedriver circuits 4504 a and 4504 b may be mounted as driver circuitsformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared. Alternatively,only the signal line driver circuits or part thereof, or only thescanning line driver circuits or part thereof may be separately formedand mounted. This embodiment is not limited to the structure illustratedin FIGS. 23A and 23B.

Through the above process, a highly reliable light-emitting displaydevice (display panel) as a semiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 12

A semiconductor device of one embodiment of the present invention can beapplied to electronic paper. Electronic paper can be used for electronicappliances of a variety of fields as long as they can display data. Forexample, electronic paper can be applied to an e-book reader (electronicbook), a poster, an advertisement in a vehicle such as a train, ordisplays of various cards such as a credit card. Examples of theelectronic appliances are illustrated in FIGS. 24A and 24B and FIG. 25.

FIG. 24A illustrates a poster 2631 using electronic paper. In the casewhere an advertising medium is printed paper, the advertisement isreplaced by hands; however, by using electronic paper to which thepresent invention is applied, the advertising display can be changed ina short time. Furthermore, stable images can be obtained without displaydefects. Note that the poster may have a configuration capable ofwirelessly transmitting and receiving data.

FIG. 24B illustrates an advertisement 2632 in a vehicle such as a train.In the case where an advertising medium is printed paper, theadvertisement is replaced by hands; however, by using electronic paperto which the present invention is applied, the advertising display canbe changed in a short time with less manpower. Furthermore, stableimages can be obtained without display defects. Note that theadvertisement in a vehicle may have a configuration capable ofwirelessly transmitting and receiving data.

FIG. 25 illustrates an example of an e-book reader 2700. For example,the e-book reader 2700 includes two housings, a housing 2701 and ahousing 2703. The housing 2701 and the housing 2703 are combined with ahinge 2711 so that the e-book reader 2700 can be opened and closed withthe hinge 2711 as an axis. With such a structure, the e-book reader 2700can be operated like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 25) and graphics can be displayed on a display portion onthe left side (the display portion 2707 in FIG. 25).

FIG. 25 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, and the like may be provided onthe same surface as the display portion of the housing. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal that can be connected to various cables such as an AC adapterand a USB cable, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the e-book reader 2700 may have a function of anelectronic dictionary.

The e-book reader 2700 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

Embodiment 13

A semiconductor device of the present invention can be applied to avariety of electronic appliances (including an amusement machine).Examples of electronic appliances are a television set (also referred toas a television or a television receiver), a monitor of a computer orthe like, a camera such as a digital camera or a digital video camera, adigital photo frame, a cellular phone (also referred to as a mobilephone or a mobile phone set), a portable game console, a portableinformation terminal, an audio reproducing device, a large-sized gamemachine such as a pachinko machine, and the like.

FIG. 26A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. Images can be displayed on the display portion 9603. Here, thehousing 9601 is supported by a stand 9605.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled by an operation key 9609 of the remote controller 9610so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Furthermore, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

FIG. 26B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. Various images can be displayed on thedisplay portion 9703. For example, the display portion 9703 can displaydata of an image shot by a digital camera or the like to function as anormal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (a USB terminal, a terminal thatcan be connected to various cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although they may beprovided on the same surface as the display portion, it is preferable toprovide them on the side surface or the back surface for the design ofthe digital photo frame 9700. For example, a memory storing data of animage shot by a digital camera is inserted in the recording mediuminsertion portion of the digital photo frame, whereby the image data canbe downloaded and displayed on the display portion 9703.

The digital photo frame 9700 may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired image data can be downloaded to be displayed.

FIG. 27A is a portable amusement machine including two housings, ahousing 9881 and a housing 9891. The housings 9881 and 9891 areconnected with a connection portion 9893 so as to be opened and closed.A display portion 9882 and a display portion 9883 are incorporated inthe housing 9881 and the housing 9891, respectively. In addition, theportable amusement machine illustrated in FIG. 27A includes a speakerportion 9884, a recording medium insertion portion 9886, an LED lamp9890, an input means (an operation key 9885, a connection terminal 9887,a sensor 9888 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), or a microphone 9889), and thelike. It is needless to say that the structure of the portable amusementmachine is not limited to the above and other structures provided withat least a semiconductor device of the present invention may beemployed. The portable amusement machine may include other accessoryequipment as appropriate. The portable amusement machine illustrated inFIG. 27A has a function of reading a program or data stored in arecording medium to display it on the display portion, and a function ofsharing information with another portable amusement machine by wirelesscommunication. The portable amusement machine illustrated in FIG. 27Acan have various functions without limitation to the above.

FIG. 27B illustrates an example of a slot machine 9900 which is alarge-sized amusement machine. In the slot machine 9900, a displayportion 9903 is incorporated in a housing 9901. In addition, the slotmachine 9900 includes an operation means such as a start lever or a stopswitch, a coin slot, a speaker, and the like. It is needless to say thatthe structure of the slot machine 9900 is not limited to the above andother structures provided with at least a semiconductor device of thepresent invention may be employed. The slot machine 9900 may includeother accessory equipment as appropriate.

FIG. 28 illustrates an example of a cellular phone 1000. The cellularphone 1000 is provided with a display portion 1002 incorporated in ahousing 1001, operation buttons 1003, an external connection port 1004,a speaker 1005, a microphone 1006, and the like.

When the display portion 1002 of the cellular phone 1000 illustrated inFIG. 28 is touched with a finger or the like, data can be input into thecellular phone 1000. Furthermore, operations such as making calls andcomposing mails can be performed by touching the display portion 1002with a finger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing a mail, a textinput mode mainly for inputting text is selected for the display portion1002 so that text displayed on a screen can be input. In that case, itis preferable to display a keyboard or number buttons on almost all thearea of the screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1000, display on the screen of the display portion 1002can be automatically switched by determining the direction of thecellular phone 1000 (whether the cellular phone 1000 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen mode is switched by touching the display portion 1002 oroperating the operation buttons 1003 of the housing 1001. Alternatively,the screen mode may be switched depending on the kind of imagesdisplayed on the display portion 1002. For example, when a signal of animage displayed on the display portion is of moving image data, thescreen mode is switched to the display mode. When the signal is of textdata, the screen mode is switched to the input mode.

Furthermore, in the input mode, when input by touching the displayportion 1002 is not performed for a certain period while a signal isdetected by the optical sensor in the display portion 1002, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1002 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken bytouching the display portion 1002 with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, by providing abacklight or sensing light source emitting a near-infrared light for thedisplay portion, an image of a finger vein, a palm vein, or the like canalso be taken.

This application is based on Japanese Patent Application serial No.2008-241335 filed with Japan Patent Office on Sep. 19, 2008, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a gate electrode; a gateinsulating layer over the gate electrode; a semiconductor layercomprising an oxide semiconductor including indium over the gateinsulating layer; and a source electrode and a drain electrode over thesemiconductor layer, wherein the semiconductor layer has a recessedportion between the source electrode and the drain electrode, andwherein an entirety of the semiconductor layer overlaps with the gateelectrode.
 2. The semiconductor device according to claim 1, wherein aside surface of the recessed portion aligns with a side surface of oneof the source electrode and the drain electrode.
 3. The semiconductordevice according to claim 1, wherein the oxide semiconductor is anIn—Ga—Zn—O based oxide semiconductor.
 4. The semiconductor deviceaccording to claim 1, further comprising: an insulating layer over thesemiconductor layer, the source electrode, and the drain electrode,wherein the insulating layer is in contact with the semiconductor layerin the recessed portion.
 5. The semiconductor device according to claim1, further comprising: an insulating layer over the semiconductor layer,the source electrode, and the drain electrode, wherein the insulatinglayer is in contact with the semiconductor layer in the recessedportion, and wherein the insulating layer comprises aluminum oxide. 6.The semiconductor device according to claim 1, further comprising: apixel electrode electrically connected to one of the source electrodeand the drain electrode, wherein the pixel electrode comprises atransparent conductive material.
 7. The semiconductor device accordingto claim 1, further comprising: a pixel electrode electrically connectedto one of the source electrode and the drain electrode, and a secondgate electrode adjacent to the gate electrode, wherein the second gateelectrode extends to form a gate wiring, and wherein the pixel electrodeoverlaps with the gate wiring.
 8. The semiconductor device according toclaim 1, wherein the source electrode is in contact with a side surfaceof the semiconductor layer, and wherein the drain electrode is incontact with a side surface of the semiconductor layer.
 9. Asemiconductor device comprising: a gate electrode; a gate insulatinglayer over the gate electrode; a first semiconductor layer comprising afirst oxide semiconductor including indium over the gate insulatinglayer; and a second semiconductor layer comprising a second oxidesemiconductor over and in contact with the first semiconductor layer; athird semiconductor layer comprising the second oxide semiconductor overand in contact with the first semiconductor layer; a source electrodeover the second semiconductor layer; and a drain electrode over thethird semiconductor layer, wherein the first semiconductor layer has arecessed portion between the source electrode and the drain electrode,and wherein an entirety of the semiconductor layer overlaps with thegate electrode.
 10. The semiconductor device according to claim 9,wherein a side surface of the recessed portion, a side surface of one ofthe second semiconductor layer and the third semiconductor layer, and aside surface of one of the source electrode and the drain electrodealign with one another.
 11. The semiconductor device according to claim9, wherein the first oxide semiconductor is an In—Ga—Zn—O based oxidesemiconductor.
 12. The semiconductor device according to claim 9,further comprising: an insulating layer over the first semiconductorlayer, the source electrode, and the drain electrode, wherein theinsulating layer is in contact with the first semiconductor layer in therecessed portion.
 13. The semiconductor device according to claim 9,further comprising: an insulating layer over the first semiconductorlayer, the source electrode, and the drain electrode, wherein theinsulating layer is in contact with the first semiconductor layer in therecessed portion, and wherein the insulating layer comprises aluminumoxide.
 14. The semiconductor device according to claim 9, furthercomprising: a pixel electrode electrically connected to one of thesource electrode and the drain electrode, wherein the pixel electrodecomprises a transparent conductive material.
 15. The semiconductordevice according to claim 9, further comprising: a pixel electrodeelectrically connected to one of the source electrode and the drainelectrode, and a second gate electrode adjacent to the gate electrode,wherein the second gate electrode extends to form a gate wiring, andwherein the pixel electrode overlaps with the gate wiring.
 16. Thesemiconductor device according to claim 9, wherein the source electrodeis in contact with a side surface of the first semiconductor layer, andwherein the drain electrode is in contact with a side surface of thefirst semiconductor layer.
 17. A semiconductor device comprising: a gateelectrode; a gate insulating layer over the gate electrode; a firstsemiconductor layer comprising a first oxide semiconductor includingindium over the gate insulating layer; and a second semiconductor layercomprising a second oxide semiconductor over and in contact with thefirst semiconductor layer; a third semiconductor layer comprising thesecond oxide semiconductor over and in contact with the firstsemiconductor layer; a source electrode over the second semiconductorlayer; and a drain electrode over the third semiconductor layer, whereinthe first semiconductor layer has a recessed portion between the sourceelectrode and the drain electrode, wherein an entirety of thesemiconductor layer overlaps with the gate electrode, and wherein thesecond semiconductor layer and the third semiconductor layer each has ahigher electric conductivity than the first semiconductor layer.
 18. Thesemiconductor device according to claim 17, wherein a side surface ofthe recessed portion, a side surface of one of the second semiconductorlayer and the third semiconductor layer, and a side surface of one ofthe source electrode and the drain electrode align with one another. 19.The semiconductor device according to claim 17, wherein the first oxidesemiconductor is an In—Ga—Zn—O based oxide semiconductor.
 20. Thesemiconductor device according to claim 17, further comprising: aninsulating layer over the first semiconductor layer, the sourceelectrode, and the drain electrode, wherein the insulating layer is incontact with the first semiconductor layer in the recessed portion. 21.The semiconductor device according to claim 17, further comprising: aninsulating layer over the first semiconductor layer, the sourceelectrode, and the drain electrode, wherein the insulating layer is incontact with the first semiconductor layer in the recessed portion, andwherein the insulating layer comprises aluminum oxide.
 22. Thesemiconductor device according to claim 17, further comprising: a pixelelectrode electrically connected to one of the source electrode and thedrain electrode, wherein the pixel electrode comprises a transparentconductive material.
 23. The semiconductor device according to claim 17,further comprising: a pixel electrode electrically connected to one ofthe source electrode and the drain electrode, and a second gateelectrode adjacent to the gate electrode, wherein the second gateelectrode extends to form a gate wiring, and wherein the pixel electrodeoverlaps with the gate wiring.
 24. The semiconductor device according toclaim 17, wherein the source electrode is in contact with a side surfaceof the first semiconductor layer, and wherein the drain electrode is incontact with a side surface of the first semiconductor layer.